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PubMed1996

Stability of maxilla downgrafting after rigid or wire fixation.

PURPOSE: This article compares the long-term outcomes of rigid internal fixation with wire fixation. PATIENTS AND METHODS: In this retrospective study, nine cases of vertical midface augmentation in which rigid fixation was used were compared with 11 cases with wire fixation. One surgeon completed all cases for the rigid fixation group, and another surgeon completed the cases in the wire fixation group. RESULTS: Follow-up was 16 +/- 11 months for the rigid fixation group and 20 +/- 12 months for the wire fixation group. Inferior movement at the anterior portion of the maxilla was 7.0 +/- 2.9 mm with rigid fixation and 4.5 +/- 3.6 mm with wire fixation (P < .05). Postsurgical superior movement (relapse) was 0.4 +/- 0.4 mm with rigid fixation and 2.4 +/- 2.4 mm with wire fixation (P < .01). Inferior movement at the posterior maxilla was 3.1 +/- 0.2 mm with rigid fixation and 2.8 +/- 2.3 mm with wire fixation. Postsurgical superior movement (relapse) was 0.8 +/- 0.4 mm with rigid fixation and 0.5 +/- 2.3 mm with wire fixation, which was not significantly different. CONCLUSION: This comparison showed downgrafting of the maxilla using autogenous bone harvested from the iliac crest and rigid internal fixation to be a predictable and stable procedure.

Adolescent↗
PubMed2004

Graft-bone motion and tensile properties of hamstring and patellar tendon anterior cruciate ligament femoral graft fixation under cyclic loading.

PURPOSE: To assess longitudinal graft-bone motion and tensile properties of the femur-anterior cruciate ligament (ACL) graft fixation-ACL graft complex based on the hypothesis that there is little difference in graft-bone motion between suspensory and aperture hamstring ACL femoral graft fixation techniques, and between hamstring and patellar tendon ACL femoral graft fixation techniques. TYPE OF STUDY: In vitro biomechanical study using human cadavers. METHODS: The distal femur-ACL graft fixation-ACL graft complex was cyclically loaded between 50 and 250 N at 1 Hz for 1,000 cycles with the direction of the load applied parallel to the axis of the femoral bone tunnel. Graft-bone motion was measured indirectly using retroreflective markers and a video motion-analysis system. Tensile testing to failure was performed at 1 mm/sec for fixation techniques completing 1,000 cycles without fixation failure. RESULTS: Among the hamstring fixation techniques, 4 of 13 Bio-Interference screws (Arthrex, Naples, FL), 2 of 12 LinX HT fasteners (DePuy Mitek, Norwood, MA), and 1 of 11 TransFix cross-pins (Arthrex) failed before completing 1,000 cycles. Five of 13 patellar tendon grafts fixed with metal interference screws, and 2 of 12 patellar tendon grafts fixed with a plastic button and No. 5 sutures failed before completing 1,000 cycles. Suture/button fixation of patellar tendon grafts resulted in significantly more graft-bone motion than hamstring tendon grafts fixed using the Bone Mulch Screw (Arthrotek, Warsaw, IN), or interference screw fixation of patellar tendon and hamstring grafts. Otherwise, there was no significant difference in graft-bone motion among the various hamstring fixation techniques or the various hamstring fixation techniques and interference screw fixation of patellar tendon grafts. Maximum graft-bone displacement after cyclic loading was significantly greater for hamstring grafts fixed with the EndoButton and EndoButton Tape (Smith & Nephew Endoscopy, Andover, MA) compared with the other hamstring fixation techniques and interference screw fixation of patellar tendon grafts. All fixation techniques except hamstring tendon grafts fixed with the Bio-Interference screw achieved at least 59% of maximum graft-bone displacement after 20 cycles. Hamstring tendon grafts fixed with the EndoButton CL were significantly stronger than all other hamstring and patellar tendon fixation methods. Patellar tendon grafts fixed with interference screws and hamstring tendon grafts fixed with interference screws and the Bone Mulch Screw and TransFix were significantly stiffer than hamstring tendon grafts fixed with the EndoButton CL. CONCLUSIONS: There is no significant difference in graft-bone motion between aperture and suspensory femoral fixation methods when the stiffness of the femur-ACL graft fixation-ACL graft complex is similar. CLINICAL RELEVANCE: The small differences in graft-bone motion reported in our study provide further evidence that graft-tunnel motion or the so-called bungee effect is unlikely to be the primary cause of radiographic bone tunnel enlargement following ACL reconstruction.

Anterior Cruciate Ligament↗
PubMed2005

Fixation topography in normal test persons.

BACKGROUND: The eye is moved so that the object of interest falls on the central fovea, where the spatial resolution is highest. In the present study we quantified eye movements of normal test persons during steady fixation and characterized the fixation using a 3D fixation plot (X horizontal eye position, Y vertical eye position, Z time in each eye position). METHOD: Fixation eye movements were quantified binocularly in ten normal test persons during a 40-s fixation task using an infrared recording technique. RESULTS: The fixation plot was characterized by a single preferred fixation locus in 17 eyes. One eye had two distinctly separated preferred fixation locations and in two eyes the configuration of fixation plot was flat with no single identifiable locus of fixation. The fixation plots were elliptical along the horizontal meridian in 9 eyes, elliptical along the vertical meridian in 8 eyes, and round in 3 eyes. The fixation area (RAF95) ranged between 1418 and 14182 arcmin(2), and a significant positive correlation was found between RAF95 and the mean microsaccadic amplitude (p<0.001). CONCLUSION: The fixation plots are often characterized by a single preferred fixation locus but may also be almost flat with no identifiable location of fixation. The individual fixations patterns resembles the cone density contour plots as found in histological studies, and it may be speculated, that the shape of the fixation plot is determined by the cone density topography.

Adult↗
PubMed2005

[Single-plane external fixation of fresh fractures of the femur: critical analysis of 53 cases].

PURPOSE OF THE STUDY: External fixation has not been widely used for femoral fractures and few series are reported in the literature. External fixation is generally reserved for severe open fractures, for vessel injury or multiple trauma with life threatening. We present a retrospective analysis of a serie treated in a single center in order to detail the indications of this fixation technique. MATERIAL AND METHODS: From 1984 to Jun 2002, 49 patients with femoral fractures were treated by external fixation. The series included 36 men and 13 women, mean age 31 years. All were victims of high-energy trauma: traffic accident (n = 40), fall from high level (n = 4), firearm wound (n = 5). Multiple fractures were present in all patients except seven and 24 patients had multiple injuries. Forty fractures were open fractures: two type 1, ten type 2, four type 3a, 23 type 3b and five type 3c in the Gustilo classification. Twenty-seven were shaft fractures and 26 involved the distal metaphyseoepiphyseal portion of the femur. Loss of cortical stock was noted in five cases and total loss of a segment in four. Surgery was deferred in 19 patients, mean six days. A single-plane external fixation was used (Orthofix) with a femorofemoral frontolatateral assembly. Transepiphyseal screw fixation was also used to stabilize the distal fracture in eleven cases. RESULTS: One patient with a bifocal fracture of the femur died from head trauma. Three patients required above knee amputation after failure of a vessel bypass or due to septic necrosis of the reconstruction flap. Five patients required a second reduction within days of external fixation. On the AP view, femoral alignment was successfully reestablished at +/- 5 degrees in 45 cases, ranged from 5 degrees to 10 degrees in seven and was greater than 10 degrees in one. On the lateral view, alignment was between 5 degrees and 10 degrees in 42 cases and greater than 10 degrees in one. Femur length was equal to the healthy side in 23 cases, and was shortened 1-2 cm in 26. Four metaphyseal fractures resulted in a 3 cm shortening. Bone healing time was available for 42 patients (1 death, 3 amputations, 3 lost to follow-up). Elective conversion to internal fixation was performed in ten patients (five lateral cortical plates and five centromedullary nailings). These patients all achieved first-intention bone healing with a mean time of 7.4 months. Exclusive external fixation was planned for 34 fractures. First-intention healing was achieved in 25 (17 shaft and 8 distal) without bone graft with an average time of 7.3 months. Ten patients had one or more osteitis foci on pin tracts. Two patients in this group developed recurrent fracture after removal of the external fixator. Nine fractures did not heal and required revision with centromedullary nailing (n = 5) or plate fixation with autograft (n = 4). Nailings for nonunion were successful but plate fixation was compromised by infection in one patient and recurrent fracture after plate removal in another. Fourteen patients underwent joint mobilization under general anesthesia and 14 had open arthrolysis. Mean follow-up was 2.8 years. Mean active flexion was 90 degrees (30-130 degrees). Ten patients exhibited flexion between 30 degrees and 60 degrees and 19 between 70 degrees and 100 degrees. Knee flexion was greater than 110 degrees in 15 patients. Residual 10 degrees flexion was noted in six knees. Mean leg length discrepancy was 0.4 +/- 0.6 after distal fracture and 0.8 +/- 1.3 after diaphyseal fracture. DISCUSSION: The indications and results of external fixation in this series are in line with reports in the literature. For diaphyseal fractures, healing is long and difficult, partly because of the insufficient mechanical properties of external fixation. The rate of infection and stiff knee is high, particularly for distal fractures of the femur. CONCLUSION: External fixation remains the only solution to stabilize certain open diaphyseal fractures or for patients with life-threatening multiple injuries. This techniques allows control of the other traumatic lesions while waiting for internal fixation. For fractures of the distal femur, external fixation can only be advocated for metaphyseodiaphyseal fractures with an intact or reconstructed epiphyseal portion.

Adolescent↗
PubMed2007

Fundus microperimetry patterns of fixation in type 2 diabetic patients with diffuse macular edema.

BACKGROUND: Recently developed fundus microperimetry (MP-1; Nidek Technologies, Srl. Vigonza PD, Italy) was used to evaluate fixation patterns and retinal sensitivity in patients who had clinically significant macular edema with a diffuse pattern and type 2 diabetes mellitus. METHODS: In a prospective, observational case series study, 84 consecutive patients with type 2 diabetes (84 eyes) who had clinically significant macular edema with a diffuse pattern were studied by means of MP-1. Main outcome measures were fixation stability and position and mean retinal sensitivities within the central 2 degrees and 8 degrees areas. Logarithm of the minimum angle of resolution (logMAR) best-corrected visual acuity (BCVA) was evaluated by means of an Early Treatment of Diabetic Retinopathy Study chart. Foveal thickness was measured by Stratus optical coherence tomography (OCT; Carl Zeiss Meditec, Inc., Dublin, CA). Age, duration of diabetes, hemoglobin A1c (HbA1c) levels, duration of symptoms, history of panretinal photocoagulation, vitreomacular adhesion, and cystoid macular changes were documented. Patients were grouped according to both position of fixation (central or eccentric) and stability of fixation (stable or unstable). Statistical analyses included independent samples t-test, binomial test, Bonferroni correction for multiple comparisons, and generalized linear model univariate analysis. RESULTS: Of 84 eyes, 44 (52.4%) had central fixation, and 40 (47.6%) had eccentric fixation. Of 84 eyes, 34 (40.5%) had stable fixation, and 50 (59.5%) had unstable fixation. In the whole sample, mean central 2 degrees retinal sensitivity +/- SD was 6.26 +/- 3.19 dB (range, 0-14.80 dB), and mean central 8 degrees retinal sensitivity +/- SD was 7.62 +/- 2.55 (range, 3.85-14.90 dB); logMAR BCVA ranged from 0.1 to 1.3 (mean +/- SD, 0.6 +/- 0.29). Mean OCT foveal thickness +/- SD was 300.77 +/- 66.93 m (range, 220-525 m). Both comparisons between groups (stable vs. unstable and central vs. eccentric) showed statistically significant differences in mean logMAR BCVA, mean central 2 degrees and 8 degrees retinal sensitivities, and mean OCT foveal thickness. Duration of symptoms was significantly longer and HbA1c levels were higher in the groups with deterioration of stability or centrality of fixation. The prevalence of cystoid macular changes was significantly higher in the groups with eccentric or unstable fixation (P < 0.001). Generalized linear model statistical analysis showed that logMAR BCVA was significantly associated with stability of fixation (P = 0.020), mean central 8 degrees retinal sensitivity (P < 0.001), and mean OCT foveal thickness (P < 0.001). The interaction between stability of fixation and mean OCT foveal thickness showed a statistically significant association with logMAR BCVA (P = 0.003). Some eyes with eccentric fixation presented with stable fixation, especially in the presence of long-lasting symptoms, and had better visual acuity than eyes with central, but unstable fixation. CONCLUSION: Visual acuity, central retinal sensitivity, foveal thickness, duration of symptoms, HbA1c levels, and presence of cystoid macular changes were strongly associated with fixation impairment in type 2 diabetic patients who had clinically significant macular edema with a diffuse pattern. Stability of fixation and foveal thickness play a major role in conditioning BCVA.

Aged↗
PubMed1993

Fixation cells in monkey superior colliculus. I. Characteristics of cell discharge.

1. We studied the role of the superior colliculus (SC) in the control of visual fixation by recording from cells in the rostral pole of the SC in awake monkeys that were trained to perform fixation and saccade tasks. 2. We identified a subset of neurons in three monkeys that we refer to as fixation cells. These cells increased their tonic discharge rate when the monkey actively fixated a visible target spot to obtain a reward. This sustained activity persisted when the visual stimulation of the target spot was momentarily removed but the monkey was required to continue fixation. 3. The fixation cells were in the rostral pole of the SC. As the electrode descended through the SC, we encountered visual cells with foveal and parafoveal receptive fields most superficially, saccade-related burst cells with parafoveal movement fields below these visual cells, and fixation cells below the burst cells. From this sequence in depth, the fixation cells appeared to be centered in the deeper reaches of the intermediate layers, and this was confirmed by small marking lesions identified histologically. 4. During saccades, the tonically active fixation cells showed a pause in their rate of discharge. The duration of this pause was correlated to the duration of the saccade. Many cells did not decrease their discharge rate for small-amplitude contraversive saccades. 5. The saccade-related pause in fixation cell discharge always began before the onset of the saccade. The mean time from pause onset to saccade onset for contraversive saccades and ipsiversive saccades was 36.2 and 33.0 ms, respectively. Most fixation cells were reactivated before the end of contraversive saccades. The mean time from saccade terminatioN to pause end was -2.6 ms for contraversive saccades and 9.9 ms for ipsiversive saccades. The end of the saccade-related pause in fixation cell discharge was more tightly correlated to saccade termination, than pause onset was to saccade onset. 6. After the saccade-related pause in discharge, many fixation cells showed an increased discharge rate exceeding that before the pause. This increased postsaccadic discharge rate persisted for several hundred milliseconds. 7. The discharge rate of fixation cells was not consistently altered when the monkey actively fixated targets requiring different orbital positions. 8. Fixation cells discharged during smooth pursuit eye movements as they did during fixation. They maintained a steady tonic discharge during pursuit at different speeds and in different directions, provided the monkey looked at the moving target.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed2004

A biomechanical comparison of supplementary posterior translaminar facet and transfacetopedicular screw fixation after anterior lumbar interbody fusion.

OBJECT: Facet screw fixation is the lowest profile lumbar stabilization method. In this study the immediate biomechanical stability provided by the two different types of fixation are compared: translaminar facet screw (TLFS) and transfacetopedicular screw (TFPS) placement after anterior lumbar interbody fusion (ALIF) using a femoral ring allograft. Both facet screw fixation types were also compared with the gold standard, transpedicular screw and rod (TSR) fixation. METHODS: Twenty-four human lumbosacral spines were tested in the following sequence: intact state, after discectomy, after ALIF, and after TLFS, TFPS, or TSR fixation. Intervertebral motions were measured by a video-based motion capture system. The range of motion (ROM) and neutral zone (NZ) were compared for each loading to a maximum of 7.5 Nm. The ROMs for stand-alone ALIFs were less than but similar to those of the intact spine, but NZs were slightly increased in all modes. The ROMs for both TLFS and TFPS fixation were significantly decreased from those of the intact spine in all modes and those of the stand-alone ALIF in flexion and extension. The TLFS and TFPS fixations significantly reduced NZs to below that of the intact spine in all modes. Compared with NZs for ALIF, both types of fixation revealed significantly lower values, except for TLFS placement in lateral bending and TFPS fixation in lateral bending and rotation. There were no significant differences between TLFS and TFPS fixation. There were also no significant differences among both TLFS and TFPS and TSR fixations, except that TFPS was inferior to TSR in lateral bending. CONCLUSIONS: Stand-alone ALIF may not provide sufficient stability. Both facet fixations produced significant additional stability and both are comparable to TSR fixation. Although TFPS fixation revealed a slightly inferior result, TFPSs can be placed percutaneously with the assistance of fluoroscopic guidance and it makes the posterior facet fixation minimally invasive. Therefore, the TFPS fixation can be considered as a good alternative to TLFS fixation.

Aged↗
PubMed1992

Failure of cementless fixation of the femoral component in total hip arthroplasty.

The basic concepts involved in cementless fixation of the total hip arthroplasty femoral component have been controversial. Some clinicians advocate fixation only of the proximal portion of the stem so that weight-bearing loads will be transferred proximally, and proximal stress shielding will be avoided. Others advocate distal fixation to ensure rigidity of fixation but concede that it will lead to proximal stress shielding and bone loss. However, clinical evidence suggests that the design of the implant is one important factor in determining stress shielding. When the stem is smooth and cylindrical distally, proximal stress relief does not occur, even when the stem is fixed tightly distally. Because femoral component loosening is the most common clinical problem with cementless total hip arthroplasty, every effort should be made to achieve fixation of the implant. Proximal and distal fixation can be achieved with most of the available implants. For tight proximal fixation, a good proximal implant design and a precise line-to-line preparation technique are critical, but aggressive broaching and interference-fit techniques result in a high rate of proximal femoral fracture during preparation and implantation. When the conditions are good for excellent proximal fixation, a flexible stem should be selected to apply a large amount of stress proximally. The stress transferred through distal fixation will be small in this situation, and the reaming technique of the femoral diaphysis does not need to be very aggressive. Approximately 10 to 20 mm of tight distal fit combined with the 0.5-mm underreaming technique provides sufficient distal fixation, and the chance of distal femoral fracture during insertion of the stem is minimum with this technique. When conditions are unfavorable for excellent proximal fixation, such as in revision hip arthroplasty, or when a proximal deformity or osteoporosis is present, a relatively rigid stem should be selected to avoid overloading proximal fixation and, thus, generating large proximal micromotion. The stress transferred through distal fixation will be large in this situation, and a relatively aggressive but well-controlled reaming technique of the diaphysis is required. Approximately 20 to 40 mm of tight distal fit combined with a 0.5-mm underreaming technique is adequate to obtain sufficient distal fixation and still avoid distal femoral fracture. Implant design features that improve proximal fixation and instrumentation features that improve bone preparation and decrease the risk of failure will broaden the indications for cementless fixation. As fixation and instrumentation improve, more flexible implants can be developed to improve bone preservation and avoid proximal stress relief.(ABSTRACT TRUNCATED AT 400 WORDS)

Biomechanical Phenomena↗
PubMed1996

Computed tomography artifacts associated with craniofacial fixation devices: an experimental study.

This study compares the artifacts caused by eight different craniofacial fixation devices in computed tomography (CT) images. Using a Teflon CT phantom model, part I of this study involved the quantitative evaluation of the X-ray absorption properties of each fixation device. Part II utilized a human cadaveric model to determine the degree to which the artifact interfered with the visualization of anatomic structures. In part I, each fixation device was secured to the surface of the phantom and then scanned. All artifacts were compared on the basis of standard deviation in CT number. The severity of the artifact was related to the physical size of the fixation device and its composition. Vitallium devices generated a greater degree of CT artifacts than titanium devices of comparable size. In part II, fixation devices were secured to the orbital rims of human cadaveric heads and then scanned. Visualization of specified anatomic structures was graded independently. The results revealed that titanium fixation devices did not cause significant bone or soft-tissue image degradation, whereas all vitallium fixation devices, except micro mesh and micro (1.0 mm) straight plates, generated an artifact that resulted in some image degradation. The extent of image degradation was related to the fixation device size. Only the thickest vitallium fixation device, mini fragmentation (2.0 mm), resulted in bony image degradation. The degree of soft-tissue image degradation decreased as the size of vitallium fixation devices decreased such that micro fragmentation (0.8 mm) and pan fixation (1.3 mm) devices interfered with soft-tissue visualization only in the immediate vicinity of the plate. The results of this study confirm the previous work of Sullivan and colleagues and Fiala and associates. The data indicate that when postoperative imaging is an important clinical consideration: (1) the fewest number of internal fixation devices should be used to achieve rigid bony fixation, (2) the proximity of fixation devices to the regions of interest should be considered at the time of fixation, (3) titanium implants produce less artifacts than vitallium implants of comparable size, and (4) vitallium micro mesh and micro (1.0 mm) straight fixation devices do not produce artifacts resulting in significant image degradation.

Artifacts↗
PubMed2001

Does optimal timing for spine fracture fixation exist?

OBJECTIVE: To evaluate the effect of timing of spine fracture fixation on outcome in multiply injured patients. SUMMARY BACKGROUND DATA: There is little consensus regarding the optimal timing of spine fracture fixation after blunt trauma. Potential advantages of early fixation include earlier patient mobilization and fewer septic complications; disadvantages include compounded complications from associated injuries and inconvenience of surgical scheduling. METHODS: Patients with spine fractures from blunt trauma admitted to an urban level 1 trauma center during a 42-month period who required surgical spine fracture fixation were identified from the registry. Patients were analyzed according to timing of fixation, level of spine injury, and impact of associated injuries (measured by injury severity score). Early fixation was defined as within 3 days of injury, and late fixation was after 3 days. Outcomes analyzed were intensive care unit and hospital stay, ventilator days, pneumonia, survival, and hospital charges. RESULTS: Two hundred ninety-one patients had spine fracture fixation, 142 (49%) early and 149 (51%) late. Patients were clinically similar relative to age, admission blood pressure, injury severity score, and chest abbreviated injury scale score. The intensive care unit stay was shorter for patients with early fixation. The incidence of pneumonia was lower for patients with early fixation. Charges were lower for patients with early fixation. Patients were stratified by level of spine injury. There were 163 cervical (83 early, 80 late), 79 thoracic (30 early, 49 late), and 49 lumbar fractures (29 early, 20 late). There were no differences in injury severity between early and late groups for each fracture site. The most striking differences occurred in the thoracic fracture group. Early fixation was associated with a lower incidence of pneumonia, a shorter intensive care unit stay, fewer ventilator days, and lower charges. High-risk patients had lower pneumonia rates and less hospital resource utilization with early fixation. CONCLUSIONS: Early spine fracture fixation is safely performed in multiply injured patients. Early fixation is preferred in patients with thoracic spine fractures because it allows earlier mobilization and reduces the incidence of pneumonia. Although delaying fixation in the less severely injured may be convenient for scheduling, it increases hospital resource utilization and patient complications.

Adult↗
PubMed2002

Stiffness characteristics of composite hybrid external fixators.

New composite hybrid fixators for fracture stabilization using single or multiple rings with monolateral bars and half-pin fixation may provide clinical advantages such as reduced bulk and easier skin access. However, the mechanical properties of these fixators are difficult to estimate from numerous design parameters. Addressing this problem the following research questions were asked: (1) Do composite hybrid fixators have similar stiffness properties to clinically proven triple-ring fixators; (2) How does the fixation mechanism affect the stiffness properties of external fixators, and, (3) Are there interferences between ring number and fixation method? An experimental study was done on simulated metaphyseal tibial fractures and stiffness of 12 fixators was measured for different loading conditions. The results showed that triple-ring fixators provide approximately 20% stiffer properties than double-ring and single-ring fixators. No influence of ring number on the AP bending properties was found and different fixation methods were associated with large differences in fixator stiffness, whereas significant interferences were found between ring number and fixation method. Although the mechanical properties investigated in the current study cannot predict directly the clinical performance of these fixators, the stiffness data provide useful information for making decisions regarding the treatment of fractures using external fixation.

External Fixators↗
PubMed2001

Structural stiffness and reducibility of external fixators placed in malalignment and malrotation.

OBJECTIVE: To determine the structural stiffness and reducibility of various external fixators placed in malalignment and malrotation. DESIGN: Uniform testing of all external fixator configurations. SETTING: Orthopaedic biomechanical laboratory. METHODS: Thirteen external fixators from different manufacturers, in a total of fifteen configurations, were studied. All external fixators were applied to a malreduction jig initially, and a subsequent anatomic reduction was then attempted. If an anatomic reduction was possible, the structural stiffness of those fixators was determined. If anatomic reduction was not possible, the external fixator was removed and reapplied to an anatomically reduced model, and then structural stiffness was determined. RESULTS: Six of the thirteen external fixator configurations allowed an anatomic reduction after placement on a malreduction model. The other nine external fixator configurations would not allow for an anatomic reduction. All the external fixator configurations were biomechanically tested in anteroposterior bending, lateral bending, axial load, and torsion. Each fixator had its own structural stiffness and is reported. CONCLUSIONS: Some external fixators will not allow for an anatomic reduction once placed in malalignment and malrotation without repositioning of the fixator pins. External fixator configurations (i.e., single-pin, dual-pin, and multipin barclamps) affect structural stiffness. Structural stiffness widely varied among the external fixators. Proper external fixator selection will enable early fracture immobilization in malalignment and malrotation in suboptimal conditions (e.g., wartime conditions or a civilian disaster), with subsequent external fixator adjustment for an anatomic reduction.

Biomechanical Phenomena↗
PubMed1994

Biomechanical analysis of cervical stabilization systems. An assessment of transpedicular screw fixation in the cervical spine.

STUDY DESIGN: The biomechanical stability of seven cervical reconstruction methods including the transpedicular screw fixation was evaluated under four instability patterns. These four modalities, based on the range and grade of instability, allowed a reproducible biomechanical assessment to establish the in vitro role of internal fixation in the cervical spine. OBJECTIVES: This study biomechanically investigated the stability of seven reconstruction methods in the cervical spine as influenced by four instability patterns and assessed whether three-column fixation for the cervical spine using transpedicular screw fixation would provide increased stability over that of conventional cervical fixation systems. METHODS: A total of 24 calf cervical spine specimens were divided into four experimental groups. The spinal constructs including seven reconstruction techniques--the posterior AO titanium reconstruction plate, Bohlman's posterior triple-wiring, transpedicular screw fixation, anterior iliac bone graft, anterior AcroMed plate, anterior AO titanium locking plate, and combined fixation with the AO anterior plate and posterior triple-wiring--were tested under four loading modes. RESULTS: Anterior plating methods provided less stability than that of posterior constructs under axial, torsional, and flexural loading conditions. Exclusive posterior procedures provided increased stability compared with the intact spine in one level fixation, however, did not sustain the torsional stability when the anterior and middle column was eliminated in two-level fixation. The stabilizing capabilities of both the combined fixation and transpedicular screw fixation were clearly demonstrated in all loading modes, however, those of the latter were superior in multilevel fixation. CONCLUSION: Front and back approaches, employing the anterior plate and posterior triple-wiring, and transpedicular screw fixation demonstrated clear biomechanical advantages when the extent of instability increased to three-column or multilevel. Three-column fixation for the cervical spine using transpedicular screw fixation offers increased stability over that of conventional cervical fixation systems, particularly in multiple level constructs.

Animals↗
PubMed2003

Transdiscal L5-S1 screws for the fixation of isthmic spondylolisthesis: a biomechanical evaluation.

The current study is a biomechanical study using a cadaveric model of L5-S1 spondylolisthesis. The purpose of the current study was to compare, in a cadaveric model of simulated L5-S1 spondylolisthesis, the biomechanical stiffness of transdiscal fixation with traditional pedicle screw fixation, and transdiscal fixation with combined interbody/pedicle screw fixation. The surgical management of L5-S1 spondylolisthesis is a challenge because of the difficulties in achieving a reliable arthrodesis in the face of high mechanical forces. A method of lumbosacral fixation that has been used successfully in moderate grades of spondylolisthesis at our institution involves the use of transdiscal S1 pedicle screws. With this technique, S1 pedicle screws are placed through the S1 pedicle, through the superior endplate of S1, through the inferior endplate of L5, to terminate in the L5 body. Eighteen fresh human cadaveric (age 59-88 years) L5-S1 motion segments were obtained. The end of each intact motion segment was potted up to its midbody in a 10-cm-diameter polyvinylchloride end-cap using dental cement. The intact specimen was then biomechanically tested as follows: 1) axial compression (500 N), 2) flexion (10 Nm), 3) extension (10 Nm), 4) right lateral bending (10 Nm), and 5) left lateral bending (10 Nm). Stiffness values were calculated from the load-deflection curves obtained. Spondylolisthesis was then simulated by displacing L5 on S1 (% slip average = 41.3%) after performing a radical L5-S1 discectomy, L5 laminectomy, and bilateral L5-S1 facetectomies. The 18 motion segments were divided into two groups. Group I (n = 10) was biomechanically tested (as above) after pedicle screw fixation and again after replacing the S1 pedicle screws with transdiscal screws. Group II (n = 8) was biomechanically tested (as above) after combined interbody/pedicle screw fixation and again after fixation with transdiscal screws. Load-deflection curves were obtained each time, and stiffness values were calculated from the curves. Transdiscal fixation was 1.6-1.8 times stiffer than pedicle screw fixation (p < 0.05) in all loading modes tested. There were no differences in stiffness between transdiscal fixation and combined interbody/pedicle screw fixation. In a cadaveric model of simulated L5-S1 spondylolisthesis, transdiscal L5-S1 fixation produced a 1.6-1.8 times stiffer construct than traditional pedicle screw fixation. Further, the stiffness of the transdiscal fixation was equal to that of a combined interbody/pedicle screw fixation.

Aged↗
PubMed2000

Comparison of visual field progression in patients with normal pressure glaucoma between eyes with and without visual field loss that threatens fixation.

AIM: To compare the frequency and site of visual field progression and changes in visual acuity in patients with normal pressure glaucoma (NPG) with and without pre-existing visual field loss. METHOD: Patients with normal tension glaucoma were selected who had at least 10 visual fields over 5 or more years of follow up and no other condition that might influence the visual field or visual acuity. Alternate left and right eyes were selected from patients in random order. These eyes were then subdivided according to visual field defect threatening fixation, visual field defect not threatening fixation, and no visual field defect (fellow eyes). Eyes were defined as showing a threat to fixation according to the presence of a visual field defect involving one of more of four paracentral visual field locations. Pointwise linear regression analysis was applied to each visual field series using PROGRESSOR software. Progression of visual field loss was defined as the appearance of a regression slope 1 dB per year or more with a significance of p<0.01, which remained consistent with the addition of two of three successive visual fields to the series. The number of patients showing progression and the number where progression occurred in one of the four paracentral visual field locations was noted. The number of eyes losing two or more lines of Snellen visual acuity over the follow up period was also noted. RESULTS: 174 eyes of 174 patients were selected. 106 eyes had visual field loss threatening fixation, 46 eyes had visual field loss that did not threaten fixation, and 22 were fellow eyes with normal visual fields. The median follow up was 7.2 years. Eight eyes (36.4%) in the "normal visual fields" group, 31 eyes (67.4%) in the "visual field loss away from fixation" group, and 87 eyes (82.1%) in the "threat to fixation" group showed progression in any part of the visual field. Two eyes (9.1%) in the "normal visual fields" group, nine eyes (19.6%) in the "visual field loss away from fixation" group, and 45 eyes (42.5%) in the "threat to fixation" group showed progression at "threat to fixation". The Cox proportional hazards regression model showed an increased risk of progression at any part of the visual field for female sex and a decreased risk for eyes with normal visual fields. For progression at threat to fixation this model showed an increased risk with pre-existing threat to fixation. Eyes from older patients and those that went on to have progressive visual field loss at fixation were more likely to lose two lines of Snellen visual acuity over the follow up period. CONCLUSION: Since 20-30% of previously field damaged eyes and over 60% without prior field loss fail to demonstrate progressive visual field damage over a long follow up it is recommended that normal pressure glaucoma patients be monitored for progression and that potentially harmful therapy be withheld until progression is demonstrated. Although the presence of visual field loss that threatens fixation does not constitute an increased risk of visual field progression it does indicate an increased risk of further loss of visual field close to fixation which is in turn associated with loss of central acuity. In the light of this finding, patients with visual field loss that threatens fixation should be managed more aggressively.

Aged↗
PubMed2001

Harvested porcine mitral xenograft fixation: impact on fluid dynamic performance.

BACKGROUND AND AIM OF THE STUDY: Recent developments suggest that stentless bioprosthetic mitral valve heterografts should be considered in order to optimize valve hydrodynamics. The fixation process alters the mechanical properties of tissue. This study investigates the changes in mitral valve morphology and hemodynamic performance following fixation. METHODS: Porcine mitral valves were excised and attached to a physiological annular ring. Mitral valve function was studied in vitro with a rigid transparent left heart model, allowing transverse and sagittal views. Initial experiments were performed with fresh valves under physiological conditions. Three different papillary muscle positions were used, and each was recorded. After glutaraldehyde fixation, genipin fixation, and cryopreservation, the valves were re-studied while maintaining cardiac output. Performance characteristics before and after fixation were obtained from hydrodynamic pressure and flow data, high-speed video camera, digital video, Doppler ultrasound, and three-dimensional papillary muscle force measurements. Morphology changes were detected by detailed anatomic measurements of the valves before and after fixation. RESULTS: Valve length was reduced by 18.5% after fixation with genipin (p <0.001), but not with glutaraldehyde. Cryopreserved valves showed no statistically significant changes in morphology or hydrodynamic performance after preservation. The forward flow opening area was reduced by 12.2% (p <0.001) after glutaraldehyde fixation, and by 32.3% (p = 0.004) after genipin fixation. Thus, maximal forward flow velocity was increased by 33.3% (p = 0.008) after glutaraldehyde fixation and by 52.8% (p = 0.001) after genipin fixation. The flow acceleration was consistent with a funnel shape of the fixed valves causing important flow contraction beyond the orifice (vena contracta). The papillary muscle force increased with apically posterior papillary muscle displacement by 20.4% (p = 0.001) and 101.5% (p <0.001) after glutaraldehyde and genipin fixation, respectively, and total regurgitant volume was increased by 91.6% (p <0.001) and 117.3% (p <0.001), respectively. The work required by the heart simulator to maintain a constant cardiac output at constant vascular resistance increased by 24.2% (p = 0.003) and 34.2% (p = 0.004) after glutaraldehyde and genipin fixation, respectively. CONCLUSION: The present study shows that chemical fixation of porcine mitral valves adversely affects the hemodynamics of the valves, increasing overall workload. The effects were more severe after fixation with genipin than with glutaraldehyde. This suggests the need to explore other fixation agents to optimize valvular cardiac function. Cryopreservation had no detrimental effects on valvular hemodynamic performance.

Animals↗
PubMed2002

Tensile strength of mesh fixation methods in laparoscopic incisional hernia repair.

BACKGROUND: Fixation of the mesh is crucial for the successful laparoscopic repair of incisional hernias. In the present experimental study, we used a pig model to compare the tensile strengths of mesh fixation with helical titanium coils (tackers) and transabdominal wall sutures. METHODS: Thirty-six full-thickness specimens (5 x 7 cm) of the anterior abdominal wall of nine pig cadavers were randomized for fixation of a polypropylene mesh (7 x 7 cm) by either tackers or transabdominal wall sutures. The number of fixation points varied from one to five per 7-cm tissue length, with distances between fixation points of 2.3, 1.8, 1.4, and 1.2 cm, respectively. The force required to disrupt the mesh fixation (tensile strength) was measured by a dynamometer. Statistical analysis was performed using the Wilcoxon test and the Spearman rank correlation test. RESULTS: The mean tensile strength of mesh fixation by transabdominal sutures was significantly greater than that by tackers for each number of fixation points: 67 N vs 28 N for a single fixation point (p <0.001), 115 N vs 42 N for two fixation points (p <0.001), 150 N vs 63 N for three fixation points (p <0.05), 151 N vs 73 N for four fixation points (p <0.05), and 150 N vs 82 N for five fixation points (p <0.05). Increasing the number of fixation points over three per 7 cm (distance between fixation points of 1.8 cm) did not improve tensile strength. CONCLUSION: The tensile strength of transabdominal sutures is up to 2.5 times greater than the tensile strength of tackers. Therefore, the use of transabdominal sutures for mesh fixation appears to be preferable for laparoscopic incisional hernia repair.

Abdominal Wall↗
PubMed2005

Evaluation of adhesion formation, mesh fixation strength, and hydroxyproline content after intraabdominal placement of polytetrafluoroethylene mesh secured using titanium spiral tacks, nitinol anchors, and polypropylene suture or polyglactin 910 suture.

BACKGROUND: The purpose of this study is to evaluate fixation methods for polytetrafluoroethylene (ePTFE) mesh with an in vivo model of laparoscopic ventral hernia repair. METHODS: In 40 New Zealand white rabbits, a 4 x 4-cm ePTFE mesh (n = 80, two per animal) was attached to an intact peritoneum with polyglactin 910 (PG 910) (n = 20) or polypropylene (PP) (n = 20) suture, titanium spiral tacks (TS) (n = 20), or nitinol anchors (NA) (n = 20). Mesh was harvested at 8 and 16 weeks for fixation strength testing, adhesion assessment, and collagen (hydroxyproline) content. Fixation strength on day 0 was determined with mesh attached to harvested abdominal wall. Statistical significance was determined as p < 0.05. RESULTS: There was no difference in fixation strength between PP (39.1 N) and PG 910 (40.0 N) sutures at time zero. At week 8, PP (25.7 N) was significantly stronger (p < 0.05) than PG 910 (11.4 N) suture, but not at week 16. The fixation strength of TS and NA (day 0, 15.4 vs 7.4 N; week 8, 17.5 vs 15.3 N; week 16, 19.1 vs 13.8 N) was not significantly different. Fixation with PP suture was significantly (p < 0.05) stronger than that with TS and NA at day 0 (39.1, 15.4, and 7.4 N, respectively) but not at weeks 8 or 16. The fixation strength of suture decreased significantly (p < 0.05) from day 0 to week 16 (PP: day 0 = 39.1 N, week 8 = 25.7 N, week 16 = 21.4 N; PG 910: day 0 = 40.0 N, week 8 = 11.4 N, week 16 = 12.8 N). The fixation strength of NA and TS did not change significantly (NA: day 0 = 7.4 N, week 8 = 15.3 N, week 16 = 13.8 N; TS: week 0 = 15.4 N, week 8 = 17.5 N, week 16 = 19.1 N). There were no differences in adhesion area based on fixation device used; however, there were more (p < 0.05) mesh samples using NA with adhesions compared to TS and adhesion tenacity was greater (p < 0.05) compared to that of TS, PP, and PG. Hydroxyproline content at weeks 8 and 16 was similar for all fixation devices. CONCLUSIONS: The initial fixation strength for nonabsorbable suture is significantly greater than that of the metallic fixation devices, but after 8 weeks there is no difference. Laparoscopic ventral hernia repair without transabdominal suture fixation may be predisposed to acute failure. The metallic devices have similar fixation strength, although the incidence of adhesions and tenacity of adhesions appear to be greater with the nitinol anchors. Since these devices have similar fixation strengths and most likely provide adequate supplementation to transabdominal sutures for mesh fixation after laparoscopic ventral hernia repair, their use should be based on other factors, such as their propensity for adhesions, ease of application, and cost.

Alloys↗