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Commentary & Perspective | ||||||||
Commentary & Perspective on
Dislocation is reported as a complication of primary and revision total hip arthroplasties in nearly all large series of these procedures. Alberton et al. documented a dislocation rate of 7.4% in a series of 1548 revision total hip arthroplasties. This dislocation rate is considerably higher than the rate typically reported after primary total hip arthroplasty1-3. However, many factors that lead to postoperative instability are more common in revision compared with primary total hip arthroplasty. These factors include increased soft-tissue laxity, a higher incidence of greater trochanteric nonunion, osseous impingement during the range of motion, and technical difficulty in correcting limb-length discrepancy and femoral offset. The authors identified three factors that significantly affected the dislocation rate after revision total hip arthroplasty: an elevated rim liner reduced the dislocation rate (p < 0.05) while the presence of trochanteric nonunion (p < 0.001) or use of a 22-mm-diameter femoral head (p < 0.05) were associated with an increased dislocation rate. Trochanteric nonunion reduces soft-tissue tension, and use of a larger femoral head increases the impingement-free range of motion between the femoral neck and the acetabular liner. Prosthetic impingement is influenced by the position of the acetabular component, the head-neck-diameter ratio, neck geometry, and the use of an elevated rim liner4. Skirted femoral heads increase the relative diameter of the femoral neck and reduce the range of motion to impingement. When skirted femoral heads are needed to increase neck length, it may be appropriate to use a femoral head with a larger rather than a smaller diameter to maintain an adequate head-neck-diameter ratio. The data presented by Alberton et al. provide compelling evidence for the routine use of elevated rim liners in revision total hip arthroplasty, but these liners must be placed accurately. Although they increase coverage of the femoral head, they also reduce the range of motion, leading to impingement in other planes. Therefore, the range of motion to impingement should be carefully assessed intraoperatively with use of trial components before the final components are selected. Anterior impingement leading to posterior dislocation should be checked with the hip in combined flexion, adduction, and internal rotation. Posterior impingement leading to anterior dislocation should be checked with the hip extended and externally rotated. Placing the elevated rim posteriorly may result in posterior impingement, particularly if the femoral stem and acetabular cup are excessively anteverted. In this instance, use of a neutral liner or repositioning of the elevated rim liner superiorly may be more appropriate. Some elevated rim liners are anteverted, which has the same effect as further anteversion of the metal shell, while others have a rim elevated over a portion of the liner to maintain a greater range of motion to impingement4. Despite optimal positioning of the implant, dislocation can still occur because of inadequate soft-tissue tension. Greater trochanteric advancement to increase soft-tissue tension, or use of a constrained cup, may be necessary to achieve stability. Poor soft-tissue tension is also a factor that can predispose the hip to dislocation after primary total hip arthroplasty. In their cadaver study, Sioen et al. compared the stability provided by two different posterior capsulotendinous repairs after total hip arthroplasty performed through a posterolateral approach. The study confirms the clinical observation that posterior transosseous capsulotendinous repair improves stability1-3. Repair of the capsule and the posterior short external rotator tendons to either the abductor tendon or the greater trochanter improved stability, although transosseous repair of the capsule to the greater trochanter provided more stability than did the soft-tissue repair to the abductor tendon. To achieve transosseous repair, adequate length of the capsulotendinous envelope, consisting of the capsule and short external rotator tendons, must be obtained to reach the greater trochanter. This may not be feasible in the setting of revision total hip arthroplasty when substantial limb-shortening due to component migration, external rotation contracture, and scar formation have altered the position of the posterior capsular and tendinous restraints. Attempts to achieve transosseous repair in this setting may result in excess tension on the repair during hip flexion and internal rotation, leading to pull-out of the sutures. Transosseous repair also requires adequate trochanteric bone stock and may not be feasible in hips with severe osteoporosis. After repair, the functional range of motion of the hip should be checked to ensure the integrity of the repair. Soft-tissue repair to the abductor tendon requires less length of the capsulotendinous flap, and may sustain less tension than would transosseous repair throughout the range of motion in the hip. Sioen et al. have clearly demonstrated the importance of capsulotendinous repair after total hip arthroplasty with use of a posterolateral approach. However, if transosseous repair is not feasible for any reason, then repair to the abductor tendon should be performed. *The author did not receive grants or outside funding in support of the research or preparation of this manuscript. He did not receive payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the author is affiliated or associated. References 1. White RE Jr, Forness TJ, Allman JK, Junick DW. Effect of posterior capsular repair on early dislocation in primary total hip replacement. Clin Orthop. 2001;393:163-7. | ||||||||
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