Biomedical subjects
Publications and source records attributed to J E Bechtold.
This article presents examples of current technologies in computer graphics as applied to foot and ankle surgery. Presently, there are two areas of great potential in computer graphics: (1) preoperative surgical planning, and (2) computer-aided design and manufacturing of custom implants.
Although infection is a serious complication associated with the use of orthopedic prosthetic implants, the impact of the metal used in these devices on host defense is poorly understood. The authors investigated the effect of stainless steel, titanium, titanium alloy, and cobalt-chromium alloy on the respiratory burst of polymorphonuclear leukocytes (PMN), a vital component of bactericidal activity. In the presence of stainless steel powder or supernatants obtained from the incubation of stainless steel in buffer, superoxide production by PMN was significantly impaired. Titanium, titanium alloy, and cobalt-chrome alloy had no significant effect on superoxide production. Nickel and chromium, the only metal ions detectable in the stainless steel supernatant, did not impair superoxide production when tested at concentrations similar to those found in the supernatant. Inhibition of PMN superoxide production may play a role in the establishment and persistence of stainless steel device-related infections.
A simple numerical technique for parametric evaluation of orthopaedic implant systems, to be used as a screening tool before complex structural analysis (e.g. Finite Element Method), is the subject of this paper. A modified Beams on Elastic Foundation model (with non-constant foundation modulus) is solved using this numerical technique based on B-spline differential equation modelling. A model with variation in the modulus of the foundation, as solved with this spline technique, was compared with a model with constant foundation modulus, solvable with closed form techniques. While deflections were smaller, the reaction force was up to ten times greater for the models with constant modulus of foundation, compared with varying modulus. The model presented in this paper is a refinement of previous models using closed form solution techniques for foundations with constant moduli. It is primarily useful for detecting trends in parametric analyses, or to select specific cases for further analysis by more computationally intensive analytic methods.
Two transpedicular spinal instrumentation systems were developed for fixation of the lumbosacral junction: a transpedicular fixator and the transpedicular screw/rod system. Mechanical testing showed that the new systems have a rigidity that is intermediate between conventional wired implants (Galveston and Luque ring) and plate systems (Steffee plate). Neither pedicular implant approached the rigidity of the Steffee plate with S2 fixation, but in compression and anterior bending both were more rigid than the Steffee plate without S2 fixation. Despite the apparent mechanical advantages of the transpedicular fixator, it is currently too bulky for clinical use. The transpedicular screw/rod system is more appealing because the size leaves adequate area for bone grafting, and device placement is technically simple. Furthermore, it may be contoured to any plane, while retaining stability provided by the clamps and screws.
Interlocking intramedullary (IM) nails allow more comminuted and proximal or distal femoral fractures to be successfully treated than previously possible with routine IM nailing. Autopsy specimens were prepared to evaluate the effectiveness of different locking mechanisms on fracture site stability. Grosse-Kempf (GK) and Brooker-Wills (BW) IM nails were inserted in anatomic specimen femurs with transverse fractures and 1-, 2-, and 3-cm defects. The femurs were loaded in four-point bending, and bending stiffness was calculated. The femurs were also loaded in torsion, and the amount of slippage between the nail and bone (at 10 Nm of applied torque) was measured. The GK nail, fully interlocked, had the lowest amount of rotational slip, followed by the BW and the GK noninterlocked nail. Bending stiffness was not significantly different for these IM nails.
This study measured the vertical displacement of three kinds of cementless tibial components [Porous Coated Anatomical (PCA), Tricon, and Whiteside], under eccentric loading up to 2,225 N. Displacement between the tibial tray and the proximal tibia was measured with linear variable differential transformers at the anterior and posterior side when anteriorly or posteriorly loaded, and at the medial and lateral side when medially or laterally loaded. The general pattern of motion was sinking at the loaded side and lift-off at the opposite side. Lift-off opposite the loaded side was fairly small for all components at all measurement sites. Among the three components, the Whiteside showed the smallest displacements. The Tricon (when anteriorly or posteriorly loaded), and the PCA (when medially or laterally loaded) showed sinking at the loaded side. Anterior screw fixation of the PCA was not effective in preventing anterior lift-off. The tilting motion of the tibial components observed in this study implies instability of the initial fixation, which could possibly compromise bony ingrowth. Furthermore, this tilting could cause uneven distribution of load, and potentially result in fracture of the underlying bone.
Mechanical testing protocols and test methods used to evaluate spinal implants have varied widely. Use of different test methods would not present a problem except that multicomponent mechanical systems, such as the spine and spinal implant constructs, yield different mechanical properties when subjected to different test methods. The goal of this article is to outline considerations specific to the mechanical assessment of spinal implants. With standard test methods and treatment of results, redundancy between testing programs will be reduced and comparison of results will be facilitated.
A long, curved-stem, uncemented femoral component was evaluated for use in primary and revision hip arthroplasty in young patients aged 43.5 years (range, 16-60 years). The femoral component has a proximal wedge shape and a medial collar for proximal press fit and load transfer; anterior and posterior ingrowth pads; anteverted head and neck; and a long, anatomically curved intramedullary stem for distal stability. In testing the contribution of the long, curved stem to the torsional stability of uncemented prostheses by comparing it with other stems, the long, curved stem was the most stable, followed by a shorter straight stem, and a short, proximally curved stem. If the long, and a short, proximally curved stem. If the long, curved stem is incrementally shortened by 1.5-7.5 cm, the initial torsional stability is reduced in proportion to the amount of shortening. Geometrically, for a given patient, the stem-bone contact (and thus the distal stability) is increased when a larger stem is inserted. Clinically, Harris hip scores in 69 hips followed for five to seven years (average patient age, 43.5 years) improved from 52.6 to 92.0 at last follow-up evaluation. The femoral component revision rate was 2.9%, the acetabular rate was 8.9%, and bipolar revision rate was 3.1%, for a combined rate of 12%.
The rotational stability of fractures of the radius and ulna treated with Rush pins and/or fracture bracing was studied in six fresh cadaver forearms. Forearm rotation and fracture site motion (rotation) were measured as functions of applied forearm torque and rotation (pronation-supination). Values were obtained from the forearm: (1) intact and with both bones fractured; (2) without fixation; (3) with a fracture brace; (4) with Rush pins; and (5) with a combination of Rush pins and a fracture brace. A brace was ineffectual in reducing fracture site motion when the wrist was rotated to specified angles compared to fracture site motion for the forearm with no fixation treatment rotated to the same angles. When loading to specific torque levels, however, the brace reduced fracture site motion to one-half the motion with no fixation treatment. Under both loading conditions, Rush pin fixation significantly and markedly reduced the fracture site motion (to approximately one-eighth of the motion with no fixation treatment), whereas a brace in conjunction with Rush pins did not significantly further reduce the fracture site motion. The radius showed more motion at fracture site than the ulna.
This study quantified changes in the cement-bone interface shear strength between primary and first- and second-revision arthroplasties as a function of mechanical interlock between the cement and bone. There were 128 segments obtained from four pairs of fresh human femora that were prepared sequentially as for primary and first and second revisions, taking care to maintain original canal morphology. Cement was pressurized into the cavity of the anatomic specimens, and the maximum interface shear strength between the cement plug and the bone was experimentally determined for each revision. First-revision interface shear strength was reduced to 20.6% of primary strength, and second revision strength to 6.8% of primary strength.
Patellar prosthesis positioning in 40 primary total knee replacements was evaluated with regard to (1) patellar tilt, (2) angle between the patellar component and patellar bony remnant, (3) lateral versus medial placement, (4) patellar height, and (5) size of the patellar component versus patellar length. Two different designs were evaluated. Twenty knees that were resurfaced with a patellar button prosthesis and implanted with conventional surgical technique constituted Group A. Twenty knees that were resurfaced with a new biconvex prosthesis and implanted with specially designed instrumentation constituted Group B. For Group A, the patellar tilt averaged 4.25 degrees preoperative and 8.35 degrees postoperative. For Group B the patellar tilt averaged 4.60 degrees preoperative and 1.83 degrees postoperative. In Group B, there was significant improvement in patellar position, with only two patients showing values exceeding the neutral range (p = 0.0409). The angle between the patellar component and the bony remnant in Group A averaged 5.975 degrees, and in Group B, 1.447 degrees. Two patients in Group A and 18 patients in Group B had values in the normal range for patellar tilt and the angle between the prosthesis and the bone. The prosthesis was inserted medial to the patellar center in six patients in Group A and one patient in Group B. A significant decrease in patellar height was noted for both groups, but values were still within the normal range. Both groups showed significant increase in the postoperative articular length, indicating that oversized prostheses were inserted and excess bone removed.
On twenty-six cemented total hip replacements that had been followed for an average of 10.4 years (range, nine to thirteen years), a retrospective radiographic study was done to assess dimensional changes in the cross section of the bone in a group of asymptomatic patients. Cortical dimensions were measured on radiographs and were normalized using a radiographic distortion factor that was derived from the width and length of the prosthesis. The data were then analyzed using a least-squares method. Analyses were performed for the men, for the women, and for the combined group. The results for all three groups showed a significant decrease in cortical thickness as well as widening of the medullary canal, but no periosteal expansion.
Implementation of newly developed computer modelling techniques and computer graphics displays and software have greatly aided the orthopedic design engineer and physician in creating a custom implant with good anatomic conformity in a short turnaround time. Further advances in computerized design and manufacturing will continue to simplify the development of custom prostheses and enlarge their niche in the joint replacement market.
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OBJECTIVE: To compare external fixator clamps from Kirschner-Ehmer (K-E), Synthes, and Meynard with respect to 6 mechanical parameters. Study Design-A bench test of mechanical properties. METHODS: Specially designed fixtures were used to mechanically test 6 clamps of each type at 2.5, 5.0, and 7.5 Newton-Meters of clamp bolt-tightening torque. RESULTS: Components slipped axially and torsionally in the K-E clamp at higher forces for all parameters except for clamp bolt axis pivot. No bolt axis pivot occurred with the Synthes clamp. Instead, the clamp plastically deformed at the fixator-pin interface. This failure occurred at a higher applied torque than the pivot torque for other clamps. The Meynard clamp withstood significantly greater force than the K-E clamp when torsion was applied to the clamp bolt axis in the clockwise direction. Pivot forces for the K-E clamp were significantly higher than the Meynard clamp in the counterclockwise direction. CONCLUSIONS: Overall, the K-E clamp was able to resist higher axial and torsional forces before slipping than the Meynard clamp or the Synthes clamp. The Synthes clamp was best able to resist torsion around the clamp bolt axis. Torsional resistance at the clamp-fixator pin and clamp-connecting bar interface was the weakest parameter of clamp mechanics. CLINICAL RELEVANCE: The ability to resist motion within a clamp is related to fracture-reduction stability. Knowledge of the mechanical properties of fixator clamps will improve a clinician's ability to apply rigid fixation.
Human patellar tendon allografts are used to replace injured anterior cruciate ligaments. They are often stored by freeze-drying or freezing before use and have been sterilized by gamma irradiation or ethylene oxide gas. Studies comparing the mechanical properties of tendons preserved by freeze-drying and sterilized by ethylene oxide have yielded conflicting results. This study examined the effects of freeze-drying and ethylene oxide sterilization on the mechanical properties of human patellar tendons obtained from 11 human cadavers; all were free of musculoskeletal disease. The 22 patellar tendons were halved longitudinally; 7 were frozen at -70 degrees C for 8 weeks and served as controls; 8 were freeze-dried; and 7 were freeze-dried and sterilized with ethylene oxide gas. Ultimate tensile stress and longitudinal strain were greater for frozen than for freeze-dried, but not greater than freeze-dried, ethylene oxide-sterilized tendons (P < 0.05). The modulus of elasticity was similar in all tendon groups. The study suggests that some in vitro mechanical properties of frozen tendons may be marginally superior to the properties of freeze-dried tendons, even after correction for size difference. Ethylene oxide gas sterilization had no adverse effect on tendon mechanical properties.