Clinical observations suggest mid-flexion instability may occur more commonly with rotating platform (RP) total knee arthroplasty (TKA), including increased revision rates and patient-reported instability and pain. We propose that increased gap laxity leads to liftoff of the lateral femoral condyle with decreased conformity between the femoral component and polyethylene (PE) insert surface leading to PE subluxation or dislocation. The objectives of this study were to define “at risk” loading conditions that predispose patients to PE insert subluxation or spinout, and to quantify the margin of error for flexion/extension gap laxity in preventing these adverse events under physiologic loading conditions. Biomechanical testing was performed on six fresh frozen cadaveric knees implanted with a posterior stabilized RP TKA using a gap balancing technique. Rotational displacement and torque were measured over time, while stiffness, yield torque, max torque and displacement were calculated using a post-processing, custom MatLab code. Revision with varying size femoral components (size 3–6) and PE insert thicknesses (10–15mm), by downsizing one step, were used to create a spectrum of flexion/extension gap mismatch. Each configuration was subjected to three loaded testing conditions (0°, 30° and 60° flexion) in balanced and eccentric varus loading, known to represent daily clinical function and “at risk” circumstances.Introduction
Methods
Total joint arthroplasty (TJA) is a high value elective orthopaedic procedure, the indications for which may vary among surgeons as well as patients. The utilization of other discretionary procedures is known to be influenced by the availability of qualified surgeons. We investigated the existence of a correlation between geographic variation in TJA utilization and the regional density of arthroplasty surgeons. The number of Medicare-funded total hip (THA) and total knee (TKA) arthroplasties performed in predetermined geographic referral regions were obtained from the Dartmouth Atlas for 2012. The number of surgeons specializing in total joint arthroplasty in each respective region was derived from the AAHKS membership list. Linear regression was used to assess the relationship between number of arthroplasties performed per 1000 Medicare beneficiaries and the number of AAHKS-members per 100,000 beneficiaries in each Hospital Referral Region (HRR). For THA in aggregate, a positive correlation was found between number of THA performed per 1,000 beneficiaries and increasing TJA surgeon density. Positive correlations were also noted when HRRs were stratified by size from 50,000 to 250,000 beneficiaries. The number of THA performed per 1,000 beneficiaries in regions with AAHKS members was greater than in regions without (4.03 vs 3.29; p=0.008). In contrast, there was no correlation between the rate of TKA utilization and HRR surgeon density, and no consistent relationship between TKA rate and HRRs stratified by size. Likewise, there was no difference in the rate of TKA between HRRs with and without AAHKS members (8.48 vs 8.84; p=0.18). The frequency of THA positively correlates with AAHKS surgeon density in all but the largest HRRs and was greater in regions with AAHKS members than in those without. Such relationships were not apparent for TKA utilization. These data may have important implications for more cost-effective utilization of THA.
