Francis W. Cooke

Treatment of the unstable intertrochanteric fracture. Effect of the placement of the screw, its angle of insertion, and osteotomy.

We created unstable intertrochanteric fractures with a large posteromedial defect in eighteen pairs of fresh-frozen femora from cadavera, and used the femora to evaluate the stability of six combinations of treatment with sliding compression screws and sideplates. The variables that were tested were high (150-degree) angle plates compared with low (130-degree) angle plates, the position of the lag-screw in the femoral head and neck (in the center of the head as seen on both anteroposterior and lateral roentgenograms compared with posteroinferiorly), and whether or not medial cortical contact had been re-established with a limited osteotomy of the greater trochanter. The femora were loaded to the point of failure. The mode of failure, maximum load to failure, and bending rigidity of each method of fixation were measured. Of the six types of treatment, the use of a 150-degree-angle plate, position of the lag-screw in the center of the head as seen on both anteroposterior and lateral roentgenograms, and osteotomy of the greater trochanter resulted in the highest mean load to failure and the greatest rigidity. Over-all, re-establishment of medial cortical contact by means of an osteotomy of the greater trochanter significantly improved the mean load to failure and the rigidity of the fixation. The higher-angle plate, although more difficult to insert, increased the mean load to failure but had no effect on rigidity. The position of the lag-screw (in the center of the head as seen on both anteroposterior and lateral roentgenograms or posteroinferiorly) had no bearing on the mean load to failure, but it significantly affected the mode of failure.

Structural properties of a novel design of composite analogue humeri models.

Background Mechanical analogue composite bone models have been used as cadaveric bone substitutes in a wide variety of biomechanical tests. The objective of this study was to compare the structural properties of two types (Third- and Fourth-Generation) of commercially available composite analogue humeri. Methods Eighteen of each generation composite analogue humeri were evaluated for flexural rigidity, torsional rigidity, and failure strength. Three tests were performed: medial–lateral four-point bending, anterior–posterior four-point bending, and external rotational torque. Results The Fourth-Generation analogue humeri performed more closely to the biological average with respect to failure strength, flexural rigidity, and torsional stiffness when compared to the Third-Generation humeri. Both the Third- and Fourth-Generation analogues were within the range of published human bone values. There was a statistically significant difference in strength in all modes of testing between the Fourth-Generation humeri and the Third-Generation humeri. Conclusion These composite analogue humeri are ideal for standardization in biomechanical analyses. The advantage of these humeri is that their variability is significantly lower than that of cadaveric specimens for all loading regimens. The widely varying results observed when comparing composite analogue humeri to cadaveric humeri might be derived from the use of different ranges of applied load, varied test methodologies, and diverse methods of computing the stiffness. Mechanical validation of whole Fourth-Generation humeri bone models would be an appropriate follow-up to this study with a direct comparison to cadaveric humeri. Clinical relevance This study validated and advanced our overall understanding of the capacity of composite analogue humeri to model the structural properties of human bone.

Salvage procedures for failed total knee arthroplasty

Revision total knee arthroplasty for aseptic loosening or following infection can pose formidable challenges to the reconstructive surgeon. In some cases, the patient will undergo a series of operations, each resulting in ever-increasing bone loss. In extreme circumstances, the end result may be amputation, arthrodesis, or permanent resection arthroplasty. Isiklar et al.1, in a study of amputation following total knee arthroplasty, found that patients had undergone an average of six operative procedures prior to having an amputation. In a review of patients who had a reinfection following reimplantation for an infection at the site of a total knee arthroplasty, Hanssen et al.2 found that patients had undergone an average of thirteen operative procedures. Although amputation or permanent resection arthroplasty results in a poor outcome that can be avoided in most patients, situations in which these options should be considered include life-threatening infection, persistent infection, irreparable soft-tissue deficiency, severe bone loss, and the wishes of the patient following multiple failed attempts at reconstruction. Resection arthroplasty may be best tolerated by patients who are willing to accept loss of ambulation, such as those who are already disabled because of multiple joint involvement. The prevalence of amputation following total knee arthroplasty has ranged from 0.02% to 0.18%, but it has been reported to be 6% in patients with a chronic infection at the site of a total knee arthroplasty1,3-5. The outcome of amputation or resection arthroplasty following total knee arthroplasty is predictably poor, with a low likelihood of ambulation. Isiklar et al.1 reported on a series of nine above-the-knee amputations in eight patients. The mean interval between the first total knee arthroplasty and the amputation was 9.7 years. Eight of the nine amputations were performed for infection with severe bone loss, and one …

Effect of Bone Block Removal and Patellar Prosthesis on Stresses in the Human Patella

Thermoelastic stress analysis was used to examine stresses on the anterior surface of patellae after patellar bone block excision for autogenous graft anterior cru ciate ligament reconstruction. Complications of anterior cruciate ligament injury often lead to degenerative changes in the knee that can require total knee joint replacement. It was hypothesized that stresses in a bone block-compromised patella may be increased even further by insertion of a patellar prosthesis. All pa tellae were first tested intact and then were retested after a sequence of surgical modifications including pa tellar prosthesis implantation, tapered bone block ex cision, square bone block excision, and both shapes of excised bone blocks with a patellar prosthesis in place. Stresses in patellae with bone blocks excised were sig nificantly greater than stresses in intact patellae. The anterior surface stress pattern in the loaded patella was significantly altered by excision of a bone block. There were no significant differences between maximum stress in patellae with tapered and square bone blocks excised. A finite element analysis showed that excision of a larger trapezoid-shaped bone block greatly in creased maximum stress levels. Insertion of a patellar prosthesis did not significantly alter stress patterns or maximum stress levels in the patella.

The influence of sequential debridement in total knee arthroplasty on the flexion axis of the knee using computer-aided navigation

The effects of osteophyte debridement, bony cuts, and soft tissue releases on the functional flexion axis of the knee can be assessed by evaluating 3D kinematics following each step of a total knee arthroplasty. Using a navigated knee system with dedicated software, the functional flexion axis (helical axis) can be determined after each step. Five paired fresh‐frozen cadaveric knees were used with a CT scan performed on each specimen identifying implanted fiducial markers. Kinematics data were recorded during each step of sequential osseous cuts and soft tissue releases for both an unloaded and loaded limb by each of three surgeons. The functional helical (flexion/extension) axis was identified for all specimens. The internal/external rotation angle (θ) of the helical axis differed from the transepicondylar axis by −8.3° to +6.7° for the unloaded condition. θ ranged from −7.2° to +7.4° with distraction. Soft tissue releases had no effect on θ; until a bony cut of the articular surface, which increased θ from −0.3° to +9.7°. Implantation of cruciate retaining prosthetic components subsequently reduced the θ range −7.3° to +4.0°. Thus, soft tissue releases had minimal effect on θ of the helical axis except for resection of the proximal tibia. Implantation of the CR prosthesis reduced è close to that of the intact knee. In a minority of knees, the helical axis did not coincide exactly with the transepicondylar axis. Interspecimen and left/right variability of θ were significant, although interinvestigator variability and an applied distraction force were insignificant.

Mechanics of Treatment

Charley Davidson did not belong to a motorcycle gang. He had never heard of Sturgis, South Dakota, and did not even have a tattoo, but he loved the rush of wind in his face as he rode his big motorcycle back and forth to work. He was a very careful rider and always wore his helmet, but he was always worried about the lack of courtesy automobile drivers often show around bikes. It was not discourtesy, however, that caused a pickup truck suddenly to turn in front of him. The ensuing collision with a road sign resulted in a severe, open fracture of Davidson’s leg. After an assessment in the emergency room indicated no other injuries, he was taken to the operating room, where the large soft tissue wound was cleaned and debrided, and his tibia stabilized with a monoplane external fixture. Will such a stabilization device allow this fracture to heal?

Porosity of Neat and Composite Bone Cement Mantles

The effect of fiber additions to bone cement on femoral cement mantle porosity was determined. Eighteen porcine femurs were implanted with a cemented prosthesis. Three cement types were used: as-received cement, cement with untreated polyethylene terephthalate fibers, and cement with treated polyethylene terephthalate fibers. Radiographs revealed all cement mantles as grade B, with slight radiolucency at the cement-bone interface. The cement mantles were sectioned at 7 levels, and porosity was measured at each level. All specimens had similar porosities, with an overall mean percentage of porosity of 3.3%+/-2.2% and a mean pore count of 208+/-160 per section. The high pore count and porosity were not visible on the standard clinical radiographs.

Stresses in Bending

Mabel Able, 80 years old, lived alone in a two-story house taking care of her own needs, such as cooking, cleaning, and tending a small garden and a flower bed. Until about two years ago she also walked around the block every day but recently has confined herself to her house and yard. Her next-door neighbor comes by to check on her every day and helps her with her grocery list, mails packages for her, and so on. On Monday morning when the neighbor went to check Mabel, she found Mabel lying on the kitchen floor, moaning and complaining of pain in her right hip. The neighbor could see that the leg was turned out and shortened, and she immediately suspected that Mabel had slipped on the linoleum when she came downstairs to make her morning coffee. The rescue squad was called, and Mabel was transported to the hospital, where the suspicion of a fractured hip was confirmed by a radiograph. After her family doctor came in and pronounced Mabel fit for surgery, she was taken to the operating room, where internal fixation with a compression screw and side plate was accomplished without any complications. The pre- and intra-operatice X rays are shown in Figure 4.1. What were the mechanical factors c ausing Mabel’s hip to break?

Mechanics of Implants

Charley Davidson’ s pal Sanza Time had a similar accident, but, instead of fracturing his tibia, he sustained a closed fracture of the femur. This fracture was treated by closed intramedullary rodding, and Sanza, against the advice of his surgeon, was back on his bike as soon as the repair shop pronounced it ready. Also against the advice of his doctor, he went back to his job in a steel mill, where he was on his feet all day, and even played a little Softball on weekends. His doctor was hardly surprised, then, that, about 11 weeks after his operation, Sanza had a sudden pain in the thigh. A roentgenogram showed a broken rod associated with an ununited femur. How could such a thing happen?

Modeling crack extension in chopped-fiber composites

A finite element model for predicting the effect of fiber bridging, fiber properties, and fiber-matrix interface strength on the crack-tip stresses and crack propagation potential of a chopped-fiber composite is proposed.The method of virtual crack extension was used to model crack growth in a micromechanics composite material model. The purpose of this study was to investigate the effects of fiber bridging of a crack and fiber-matrix interface strength on the stress intensity and strain energy release rate in virtual crack extension. A model of an aligned, 1% by volume fiber loading, chopped-fiber composite with a preexisting crack was developed to represent a portion of a fracture toughness or fatigue crack propagation specimen. Nonlinear contact elements were used to model fiber-matrix interface strengths. The von Mises stress at the crack tip was calculated for each configuration before crack extension, and the strain energy release rate was calculated for each crack step. The presence of fibers without bridging of the crack did not greatly affect the stress at the crack tip. However, fiber bridging of the crack reduced the crack tip stress by a factor of seven. The magnitude of strain energy release rate was greatly reduced and the sign of the slope of the strain energy release rate versus crack length curve was changed from positive to negative by fiber bridging. In accordance with the theory of tough fiber reinforcement of brittle matrices, the results of applying nonlinear contact elements with varying coefficients of friction predict that an intermediate fiber-matrix interface strength will be most effective in toughening a brittle composite.