The Design of the Total Knee Prosthesis Affects the Intra-operative Joint Gap

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  • PS型人工膝関節全置換術における関節面デザインの違いによるインプラントGapの比較
  • PSガタ ジンコウ シツカンセツ ゼン チカンジュツ ニ オケル カンセツメン デザイン ノ チガイ ニ ヨル インプラント Gap ノ ヒカク

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Abstract

Objective: Soft-tissue balancing of the knee is fundamental to the success of total knee arthroplasty (TKA). The most important clinical goal in TKA is the preparation of equal and rectangular extension and flexion joint gaps because this facilitates the functional restoration of the knee. However, particular attention should be paid to the design of the articulating surface of the total knee prosthesis because it may adversely influence the joint gap. It has always been difficult to accurately assess the intra-operative joint gap during trial reduction of the components. The purpose of this study was to compare the intra-operative joint gap of two posterior-stabilized (PS) prostheses having different shapes of articulating surface (namely, fixed-bearing PS prostheses and mobile-bearing PS prostheses) with the use of a unique tensor device.<br>Methods: We measured the intra-operative joint gap in 65 knees in patients undergoing PS TKA. First, the femoral and patellar trial components of a fixed-bearing prosthesis were inserted. The joint gap was measured using a tensor device having the same shape as the fixed-bearing polyethylene insert (FB-PS gap). The center width and asymmetry (tilting) under 120 N of distracting force were measured at 0°, 30°, 45°, 60°, 90°, 120° and maximum flexion of the knee with the patellofemoral joint reduced and the quadriceps tendon sutured. Second, the femoral and patellar trial components were changed to mobile-bearing prostheses having the same chamber shape but different articulating surfaces. The joint gap was also measured using a tensor device having the same shape as the mobile-bearing polyethylene insert (MB-PS gap).<br>Results: From 30° to 120° flexion of the knee, the MB-PS gap tended to be smaller than the FB-PS gap. At 30°, 45°, and 60° flexion, the MB-PS gap was significantly smaller than the FB-PS gap.<br>Conclusion: This study showed that the joint gap was largest at 30° flexion of the knee in both PS prostheses. This may result in mid-flexion instability of PS prostheses. However, the joint gap at 30°, 45°, and 60° flexion of the MB-PS prosthesis was significantly less than that of the FB-PS prosthesis. These results suggest that the design of the total knee prosthesis affects the intra-operative joint gap and could reduce mid-flexion instability.

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