Galante Jo

Wear patterns on retrieved polyethylene tibial inserts and their relationship to technical considerations during total knee arthroplasty

Fifty-five unconstrained polyethylene tibial inserts were retrieved at revision total knee arthroplasty and examined for evidence of wear after a mean implantation time of 34.2 months (2.5-80 months). Twenty inserts were ultra-high molecular weight polyethylene (UHMWPE) and 35 were carbon-reinforced polyethylene. Topographic maps of the articular and metal-backed surfaces of each component were constructed to characterize the extent and location of polyethylene degradation, identified visually by mode. In 32 of the retrieved inserts, pre- and postarthroplasty or prerevision radiographs were analyzed for component positioning, sizing, and extremity alignment. These factors then were compared with the patterns and severity of polyethylene wear on the inserts to establish correlations. Severe generalized articular wear was seen in inserts with third body wear from patellar metal-backed failure and cement debris. Severe localized delamination wear was seen in inserts with rotational-subluxation patterns of wear (p = 0.05). The external rotation subluxation wear pattern was strongly associated with knees that had lateral subluxation of the patella (p = 0.0002). Articular wear and cold flow into screw holes tended to be greater in the tightest prearthroplasty compartment (medial in the varus knee [p = 0.0157]; lateral in the valgus knees [p = 0.0226]). Fourteen of 16 knees with a preoperative varus deformities--even when corrected to a normal postarthroplasty anatomic axis--still had greater medial compartment articular wear (p = 0.001). Twelve of these knees did not have a medial release at the time of initial arthroplasty. Preoperative varus also was found to be related to the occurrence of posteromedial cold flow of polyethylene into tibial tray screw holes (p = 0.007). Increasing tibial insert posterior slope was associated with increasingly posterior articular wear track location (p = 0.03). This study indicates that unconstrained tibial component wear patterns and severity may be associated with clinical and mechanical factors under the surgeons control, including component size and position, and knee alignment and ligament balance.

Functional adaptation and ingrowth of bone vary as a function of hip implant stiffness

The purpose of the present study was to test the hypothesis that cortical bone loss, trabecular bone density and the amount of bone ingrowth vary as a function of stem stiffness in a canine cementless hip replacement model. The study was motivated by the problem of cortical bone atrophy in the proximal femur following cementless total hip replacement. Two stem stiffnesses were used and both designs were identical in external geometry and porous coating placement. The high stiffness stem caused approximately 26% cortical bone stress-shielding and the low stiffness stem caused approximately 7.5% stress-shielding, as assessed by beam theory. Each group included nine adult, male canines who received unilateral arthroplasties for a period of six months. The animals with the low stiffness stems tended to lose less proximal cortical bone than the animals with high stiffness stems (4% +/- 9 as opposed to 11% +/- 14), but the difference was not statistically significant (p = 0.251). However, the patterns of bone ingrowth into the implant and change in medullary bone density adjacent to the implant were fundamentally different as a function of stem stiffness (p < 0.01). Most importantly, while the high stiffness group had peaks in these variables at the distal end of the stem, the low stiffness group had peak values proximally. These different patterns of functional adaptation are consistent with the idea that reduced stem stiffness enhances proximal load transfer.

Primary cementless acetabular reconstruction in patients younger than 50 years old. 7- to 11-year results.

The efficacy of primary cementless acetabular reconstruction in patients younger than 50 years of age was analyzed in 79 consecutive cementless, hemispheric, porous coated acetabular reconstructions (Harris-Galante-I). The average age was 37 years at surgery (range, 20-49 years). The average followup was 106 months (range, 78-126 months). No acetabular reconstructions were revised for aseptic loosening. Two stable acetabular reconstructions were revised during femoral revision. Two excessively worn polyethylene liners were exchanged and one acetabular osteolytic area was debrided and grafted; these procedures retained the metal shell. At final followup, all 72 acetabular reconstructions were radiographically stable. Acetabular osteolysis occurred in five cases (7.4%), from 84 to 104 months. Acetabular or femoral osteolysis occurred in patients with increased polyethylene wear. Polyethylene wear was inversely related to the patients age. Using revision and loosening, the Kaplan-Meier 10 year survival of the acetabular reconstruction was 98.8% (95% confidence interval, 96.6%-100.%). The intermediate results of cementless, hemispheric, porous coated acetabular reconstruction in younger patients was excellent with no radiographic loosening. At 7- to 11-year followup, osteolysis was the most common problem and increased in frequency and extent with continued in vivo duration.

Hybrid total hip arthroplasty: 7- to 10-year results.

One hundred fifty consecutive hybrid total hip arthroplasties in 139 patients were performed using an uncemented hemispheric porous coated acetabular component (HGP-I) with screws and a femoral component (Precoat) cemented with contemporary cementing technique. The average patient age was 67 years (range, 39-85 years). No patients were lost to followup. Eighty-six patients (91 hips) were alive for an average clinical followup of 103 months (range, 84-127 months); 81 hips had corresponding radiographic analysis. The average Harris hip score preoperatively was 47 points and increased to 88 points at followup. Ninety-five percent of patients had absent or slight pain. Aseptic loosening occurred in 2 femoral components (1.3%), 1 of which was revised for secondary osteolysis. Both hips had suboptimal cement mantles (C-2 or D grades). No femoral osteolysis was seen in stable components. Two acetabular components migrated; 1 secondary to preoperative irradiation osteonecrosis and 1 secondary to a bulk autogenous graft. Acetabular osteolysis without loosening developed in 2 patients (1.3%). Using revision and radiographic loosening as the end point, the probability of both components surviving 10 years was 96.9%, 98.6% for the acetabular component, and 98.4% for the femoral component. These results show that hybrid total hip arthroplasty offers excellent clinical function and exceptional 10-year survivorship.

Maintenance of Proximal Cortical Bone with Use of a Less Stiff Femoral Component in Hemiarthroplasty of the Hip without Cement. An Investigation in a Canine Model at Six Months and Two Years

A canine model of hemiarthroplasty of the hip was used to determine if the use of a less stiff femoral stem can reduce the amount of bone loss induced by stress-shielding. Two types of stem were used: the stiffer stems were made of a titanium alloy, and the less stiff stems were composed of a cobalt-chromium-alloy core with an outer polymer layer. The stems were identical in shape, and both types were circumferentially coated along their entire length (except for the distal five millimeters) with commercially pure titanium fiber metal. Ten dogs with each type of stem were followed for six months, and twelve dogs with each type of stem were followed for two years. Loss of cortical bone from the proximal part of the femur was associated with both types of stem, but typically 50 per cent less bone was lost with the less stiff implants. Most of the cortical loss occurred at the subperiosteal surface. The amount of medullary bone adjacent to the proximal and distal aspects of both types of stem increased; the less stiff stems were associated with a greater increase in the proximal region, and the stiffer stems were associated with a greater increase in the distal region. Similarly, there were peaks in the amount of bone growth into the proximal and distal portions of both types of stem, with a greater peak in proximal bone growth into the less stiff stems and a greater peak in distal bone growth into the stiffer stems. CLINICAL RELEVANCE: The data indicate that an effective means of promoting proximal transfer of load from the implant to the host femur is to reduce the stiffness of the stem. This is a design-related mechanism to decrease stress-shielding, thereby suppressing the loss of bone in the proximal part of the femur following hemiarthroplasty of the hip performed without cement. Such bone loss can eventually lead to loosening of the component and make subsequent reconstruction difficult.

Experimental studies of bone remodeling in total hip arthroplasty.

Experimental studies of bone remodeling after total hip arthroplasty are reviewed. Most of the studies have been motivated by the introduction of cementless hip replacements in which porous coatings are used to provide fixation by bone ingrowth. Stress shielding is thought to be the main factor responsible for bone loss after hip replacement. Experimentally, design features related to (1) the nature of the bone-implant interface and (2) stem stiffness have been tested to determine if the effects of stress shielding can be decreased. The dominant long-term design feature controlling bone remodeling in hip replacement appears to be stem stiffness.