Mobile-bearing knee replacement: concepts and results.

In summary, if TKRs are to be performed in patients who are younger and more active than those who had the initial procedures in the 1970s and 1980s, better wear performance is imperative for long-term durability, especially if surgeons continue to consider the versatility associated with modular knee-replacement systems to be a necessity. At least with some designs, including the Oxford knee and the LCS knee, the results after a minimum follow-up of 10 years are comparable with the best results after arthroplasty with fixed-bearing designs in terms of wear, loosening, and osteolysis (Table 7). As with fixed-bearing designs, there are additional challenges in terms of optimizing bearing-surface conformity and improving kinematics. Improvements in future designs of mobile-bearing total knee replacements should include better control of bearing mobility patterns to reduce the prevalence of the abnormal kinematic motions that have been observed in fluoroscopic evaluations.

Mechanical comparison of a distal femoral side plate and a retrograde intramedullary nail.

OBJECTIVE: To compare quantitatively the axial and torsional stiffness of a retrograde intramedullary nail and a fixed angle screw side plate in treating a supracondylar femur fracture in osteopenic femora. To determine the modes of failure of an intramedullary nail and a side plate under axial loading. DESIGN: Matched pair cadaveric study. SETTING: Orthopaedic biomechanics laboratory. PATIENTS AND OTHER PARTICIPANTS: Eleven matched pairs of preserved human femora were selected. The cadaveric specimens were harvested from relatively elderly donors with an average age of 75.6 years, which represents the principal population at risk for poor fracture fixation. INTERVENTION: The eleven matched pairs were osteotomized to simulate segmental structural defects in the supracondylar region. One femur of each matched pair was fixed with an intramedullary nail, and the contralateral femur was fixed with a side plate. MAIN OUTCOME MEASURES: Axial and torsional stiffness values. Axial modes of failure. RESULTS: The intramedullary nail axial stiffness was 14 percent (p = 0.04) less and torsional stiffness was 17 percent (p = 0.05) less than that provided by the side plate. The axial failure of the intramedullary nail occurred distally, allowing the hardware to protrude into the articular space. The side plate also failed distally by displacing the condylar screw into a varus angulation. CONCLUSION: The mechanical advantages favor the use of the side plate if fixation stiffness is essential. The axial mode of failure occurs distally for both fixation devices.

The Coventry Award. The effects of shelf life on clinical outcome for gamma sterilized polyethylene tibial components.

Tibial component shelf life was examined as a contributory factor of the in vivo failure of gamma sterilized prosthetic knee replacements. One hundred eighty-eight Synatomic total knee replacements sterilized by gamma irradiation in air were implanted by one surgeon into 147 patients between May 1985 and December 1994. Of these, 135 knees in 105 patients with a mean followup of 5.8 years (range, 2.1-11.3 years) were included in the study. The mean shelf life of the implants was 3.6 years (range, 0.1-10.7 years). Clinical failure for this study was defined as component retrieval resulting from polyethylene degradation. The knee components were divided into three different groups determined by their shelf storage durations of 0 to 4 years (Group 1, 93 components), between 4 and 8 years (Group 2, 21 components), or greater than 8 to 11 years (Group 3, 21 components). Six prostheses were revised because of polyethylene degradation after a mean implantation time of 2.5 years (range, 1.1-3.8 years). The mean shelf life of these six prostheses was 8.4 years (range, 5.8-9.6 years). Five years after implantation, prostheses that had shelf lives of less than 4 years had a 100% survival rate. Those that had shelf lives of 4 to 8 years before implantation had an 88.6% survival rate, and those prostheses that had shelf lives greater than 8 to 11 years had a 79.2% survival rate.

Carry it on the bad side!

Patients with diseased hips often must carry objects while walking, yet they are rarely instructed which hand to use because little has been published on the subject. We sought to evaluate the situation mathematically by determining the hip forces that result when a load is carried in the ipsilateral versus the contralateral hand. Using a free-body diagram of single-leg supported stance, we found that when a load was carried in the contralateral hand, the resultant forces on the hip were increased considerably. Conversely, when the weight was carried in the ipsilateral hand, the forces were actually lower than when no weight was carried at all. Thus, carrying a weight on the opposite side resulted in hip forces that were substantially greater than when the weight was carried on the same side.