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At least 217 records · Page 12Linked to original sources
PubMed2001

Complications of the mandibular sagittal split ramus osteotomy associated with the presence or absence of third molars.

PURPOSE: This study compared the number of unfavorable fractures after sagittal split osteotomies (SSOs) of the mandible when third molars were present or absent. PATIENTS AND METHODS: The treatment records of 262 patients (500 SSOs) were retrospectively evaluated. Group 1 consisted of 250 SSOs and concomitant removal of impacted mandibular third molars and group 2 consisted of 250 SSOs with absence of third molars. A modified SSO technique with inferior border cuts was used on all patients, and the third molars, when present, were removed after separation of the proximal and distal segments. Rigid fixation was applied using bicortical bone screws. Bone plates with monocortical screws were additionally used to secure the free bony segments of the proximal segment in cases with unfavorable fracture. RESULTS: There were 11 (2.2%) unfavorable fractures in 500 SSO procedures. The incidence of unfavorable splits was 3.2% in group 1 and 1.2 % in group 2. In group 1, all fractures occurred in teenagers, with 7 of 8 fractures extending through the extraction socket in the distal segment. Six of the 8 fractures were associated with completely impacted third molars, and 2 involved partially impacted teeth. All 3 fractures in group 2 occurred in the proximal segment. No significant difference was seen in the amount of relapse in patients with unfavorable or favorable splits. CONCLUSIONS: The occurrence of unfavorable splits is uncommon when using a modification of the SSO that includes an inferior border osteotomy. Although more unfavorable fractures occurred in teenage patients with third molars, this had no impact on the stability of the final result.

Adolescent↗
PubMed2006

Stress relaxation at the bone-pedicle screw interface in human bone.

STUDY DESIGN: A biomechanical study using human lumbar vertebra. OBJECTIVES: To assess the effects of stress relaxation on the pullout behavior of the pedicle screw in human bone. SUMMARY OF BACKGROUND DATA: The standard pullout test is widely used to assess screw-holding power; however, the effects of incremental deformation on the pullout behavior of pedicle screws are not known in human bone. A recent study in bovine bone suggests that stress relaxation during incremental pullout significantly reduces failure loads seen in pullout testing. METHODS: Fourteen human lumbar vertebrae were instrumented with pedicle screws and tested with one of two pullout protocols: standard pullout model, a continuous withdrawal, and stress relaxation pullout model, an incremental withdrawal allowing stress relaxation. Peak loads and stiffness were calculated and relaxation data were assessed using the stretched exponential function. RESULTS: Pullout strength and stiffness were significantly lower in the stress relaxation model group (P < 0.05). The characteristic relaxation time decreased and relaxation ratio increased with the increasing deformation level. The pullout and stress relaxation parameters were found to correlate with the bone quality. CONCLUSIONS: The stress relaxation process significantly affects the mechanical and viscoelastic properties of the bone-screw interface in human cadaveric bone. Stress relaxation pullout model might be a better representative of in vivo conditions by introducing the time and incremental deformation into the in vitro biomechanical testing.

Biomechanical Phenomena↗
PubMed2002

Optimizing the biomechanical compatibility of orthopedic screws for bone fracture fixation.

Progressive loosening of bone fixation screws is a well-documented phenomenon, induced by stress shielding and subsequent adaptive bone remodeling which results in bone loss around the screw. A set of two-dimensional computational (finite element) models was developed in order to test the effect of various engineering designs of fixation screws on the predicted screw-bone stress transfer, and consequently, on the biomechanical conditions for osteosynthesis. A dimensionless set of stress-transfer parameters (STP) was developed to quantify the screw-bone load sharing, enabling a convenient rating to be given of the biomechanical compatibility of practically any given screw design according to the nature of the simulated mechanical interaction. The results indicated that newly proposed screw designs, i.e. a "graded-stiffness" composite screw with a reduced-stiffness-titanium core and outer polymeric threads and an "active-compression" hollow screw which generates compressive stresses on the surrounding bone, are expected to provide significantly better biomechanical performances in terms of the STP criteria, compared with conservative screw designs. Accordingly, the present work demonstrates that finite element computer simulations can be used as a powerful tool for design and evaluation of bone screws, including geometrical features, material characteristics and even coatings.

Biocompatible Materials↗
PubMed1975

The effect of movement on the holding power of screws in bone.

Movement between screw threads and bone inhibits bone formation, revascularization and remodeling of dead bone. Movement causes the screw to become enveloped by fibrous tissue in response to necrosis and resorption of adjacent dead cortical bone. This results in a radiologically discernible radiolucent "halo" about the screw, a certain sign of screw loosening.

Animals↗
PubMed1993

The effects of depth of penetration, screw orientation, and bone density on sacral screw fixation.

Obtaining adequate sacral fixation is an important and challenging clinical problem. Bicortical screw fixation has been used to provide improved mechanical strength. The goal of this study was to evaluate the effects of depth of penetration (unicortical versus bicortical), orientation (medial versus lateral), and bone density on the strength of the bone-screw interface. Seven-millimeter Steffee screws (Acromed, Cleveland, Ohio) were placed in human cadaveric sacra. The screws were placed either medially into the centrum of the S1 body or laterally into the sacral ala. The fixation systems used, either bicortical or unicortical, were tested on a Universal testing machine. The specimens were loaded to failure and biomechanical parameters were measured. Medially oriented screws sustained the highest load to failure. In an older population, unicortical fixation sustained load to failure and initial compliance findings were similar to those for bicortical fixation. The regional bone density in the centrum of the first sacral body was denser than that in the lateral alar region, and also provided more rigid bone screw fixation.

Aged↗
PubMed1999

Penetrating bladder injury caused by a medially placed acetabular screw.

Cancellous bone screws are frequently used to improve the early stability of HA coated acetabular components during total hip arthroplasty. Avoidance of the anterosuperior and anteroinferior quadrants is recommended for transacetabular screw placement to minimize the risk of injury to intrapelvic structures. Revision arthroplasty in rheumatoid patients presents additional hazards in that the acetabular bone is often soft and deficient, and the protective depth of obturator internus and psoas is usually reduced. I report a case of delayed, but fatal, perforation of the bladder associated with a medially placed acetabular screw during revision arthroplasty in a rheumatoid patient. Unless directed safely into the superior pubic ramus, anterior quadrant screws should be avoided in these circumstances.

Acetabulum↗
PubMedDate not supplied

A biomechanical comparison of equine third metacarpal condylar bone fragment compression and screw pushout strength between headless tapered variable pitch and AO cortical bone screws.

OBJECTIVES: To compare bone fragment compression and the mechanical pushout strength and stiffness of 6.5-mm Acutrak Plus (AP) and 4.5-mm AO cortical (AO) bone screws after stabilization of a simulated equine third metacarpal (MC3) bone complete lateral condylar fracture. STUDY DESIGN: In vitro biomechanical paired study of screw insertion variables, bone fragment compression, and screw pushout tests using a bone screw stabilized simulated lateral condylar fracture model. SAMPLE POPULATION: Six pairs of cadaveric equine MC3s. METHODS: Metacarpi were placed in a fixture and centered on a biaxial load cell in a materials testing system to measure torque, compressive force, and time for drilling, tapping, and screw insertion. Fragment compression was measured with a pressure-sensing device placed between the simulated fracture fragments during screw insertion for fragment stabilization. Subsequently, screws were pushed out of the stabilized bone fragments in a single cycle to failure. A paired t test was used to assess differences between site preparation, screw insertion, fragment compression, and screw pushout variables, with significance set at P <.05. RESULTS: Measured drilling variables were comparable for AO and AP specimens. However, the AP tap had significantly greater insertion torque and force. Mean maximum screw insertion torque was significantly greater for AO screws. For fragment compression, AP screws generated 65% and 44% of the compressive pressure and force, respectively, of AO screws. AP screws tended to have higher overall pushout strength. Pushout stiffness was similar between both screw types. CONCLUSION: The 6.5-mm tapered AP screw generated less interfragmentary compressive pressure and force but had similar pushout stiffness. Evaluation of failure patterns demonstrated that AP screws had greater pushout strength compared with 4.5-mm AO screws for fixation of a simulated complete lateral condylar fracture. CLINICAL RELEVANCE: The 6.5-mm tapered AP screw should provide ample holding strength but would provide less interfragmentary compression than 4.5-mm AO screws for repair of complete lateral condylar fractures in horses.

Animals↗
PubMedDate not supplied

In vitro pullout strength of screws inserted in adult equine third metacarpal bone after overdrilling a 4.5-mm threaded insertion hole.

OBJECTIVE: To determine and compare the in vitro pullout strength of 5.5-mm cortical versus 6.5-mm cancellous bone screws inserted in the diaphysis and metaphysis of adult equine third metacarpal (MCIII) bones, in threaded 4.5-mm cortical bone screw insertion holes that were then overdrilled with a 4.5-mm drill bit to provide information relevant to the selection of a replacement screw if a 4.5-mm cortical screw is stripped. STUDY DESIGN: In vitro pullout tests of 5.5-mm cortical and 6.5-mm cancellous screws in equine MCIII bones. SAMPLE POPULATION: Two independent cadaver studies each consisting of 14 adult equine MCIII bones. METHODS: Two 4.5-mm cortical screws were placed either in the middiaphysis (study 1) or distal metaphysis (study 2) of MCIII bones. The holes were then overdrilled with a 4.5-mm drill bit and had either a 5.5-mm cortical or a 6.5-mm cancellous screw inserted; screw pullout tests were performed at a rate of 0.04 mm/second until screw or bone failure occurred. RESULTS: In diaphyseal bone, the screws failed in all tests. Tensile breaking strength for 5.5-mm cortical screws (997.5 +/- 49.3 kg) and 6.5-mm cancellous screws (931.6 +/- 19.5 kg) was not significantly different. In metaphyseal bone, the bone failed in all tests. The holding power for 6.5-mm cancellous screws (39.1 +/- 4.9 kg/mm) was significantly greater than 5.5-mm cortical screws (23.5 +/- 3.5 kg/mm) in the metaphysis. There was no difference in the tensile breaking strength of screws in the diaphysis between proximal and distal screw holes; however, the holding power was significantly greater in the distal, compared with the proximal, metaphyseal holes. CONCLUSIONS: Although tensile breaking strength was not different between 5.5-mm cortical and 6.5-mm cancellous screws in middiaphyseal cortical bone, holding power of 6.5-mm cancellous screws was greater than 5.5-mm cortical screws in metaphyseal bone of adult horses. CLINICAL RELEVANCE: If a 4.5-mm cortical bone screw strips in MCIII diaphyseal bone of adult horses, either a 5.5-mm cortical or 6.5-mm cancellous screw, however, would have equivalent pullout strengths. A 6.5-mm cancellous screw, however, would provide greater holding power than a 5.5-mm cortical screw in metaphyseal bone.

Animals↗
PubMed2004

Biomechanical comparison of inside-outside screws, cables, and regular screws, using a sawbone model.

Our aim was to conduct a biomechanical comparison of the pull-out strengths of inside-outside (I/O) screws, cables, and bone screws to determine whether I/O screws provide greater pull-out resistance than cables or bone screws, and their effectiveness with the screw diameter. There is no remarkable biomechanical experimental study comparing the I/O technique with conventional spinal techniques. The diameter of the screw heads were also biomechanically tested to determine the optimal size that can be used. In this study, 45 blocks of 50x50x5 mm of "sawbone" (synthetic bone, model 1137, Pacific Research Laboratories, Vashon, WA, USA) were used as bone substitutes. Fifteen sets of 14-mm inside-outside Dynalok screws and nuts, 15 wire cables, and 15 bone screws were inserted into a separate sawbone block. An MTS Bionx materials testing machine was used to measure the load to failure of each implant. The mean values and standard deviations of each group were calculated and Student's t-test was used for comparison. The load to failure of the inside-outside screws was significantly greater than that of the cables (p<0.0000004) and the regular bone screws (p<0.000002). The results also revealed that increasing the diameter of the head of the screw also increases the resistance against the pull-out strengths. Thus, using a larger screw in occipitocervical stabilization provides safe and stable fixation of the occipital bone to the cervical spine. This study also proved that sawbone is a useful and reliable alternative to allogenic fresh cadaveric bone grafts or animal bones for certain biomechanical testing.

Atlanto-Occipital Joint↗
PubMed2005

Reduced bone tunnel enlargement post hamstring ACL reconstruction with poly-L-lactic acid/hydroxyapatite bioabsorbable screws.

Bone tunnel enlargement following anterior cruciate ligament (ACL) reconstruction can complicate revision surgery. This study compared postoperative tibial tunnel widening in patients who underwent arthroscopically assisted, single-incision, four-strand hamstring ACL reconstruction using a poly-L-lactic acid/hydroxyapatite blend (PLLA+HA) bioabsorbable interference screw for tibial fixation, with those in whom a plain poly-L-lactic acid (PLLA) screw was used. Thirty-four patients (13 with PLLA+HA tibial interference screw fixation and 21 with plain PLLA tibial interference screws) underwent a spiral CT scan to assess maximum tibial tunnel cross-sectional area at an average of 28.7 months follow-up. An assessment of tunnel wall sclerosis adjacent to the screw (cortication) was also made. The two groups were well matched for age, sex and graft diameters. Mean tibial tunnel enlargement in patients with PLLA+HA screws was 29.9% at average 30.9 months follow-up compared with 46% in patients with plain PLLA screw at an average 26.5 months follow-up (p=0.03). The tunnel wall adjacent to the screw appeared corticated in only 21% of patients with PLLA+HA screws (p=0.02) compared with 73% of patients with PLLA screws. The blending of HA with PLLA appears to reduce postoperative tunnel widening, and the reduced tunnel wall sclerosis seen postoperatively may indicate improved screw incorporation.

Absorbable Implants↗