Effect of Voluntary Soft Tissue Tension and Constraint after Total Knee Arthroplasty on in vivo Anteroposterior Displacement

Introduction: Proper anteroposterior (AP) joint displacement is an important indicator of good clinical outcome following total knee arthroplasty (TKA). Although anesthesia would tend to exclude the contributions of voluntary soft tissue tension, its effect on knee-joint displacement has not been studied in TKA, although it has been studied in the repair of the anterior cruciate ligament. Additionally, although only a few studies refer to the effects of geometry on in vivo AP laxity [1] or stability [2], they have examined only the effect of soft tissue condition in conscious individuals. Thus, we examined the in vivo relationship between the degree of voluntary soft tissue tension and constraint after TKA and AP joint displacement simultaneously and clarified the interaction between the two variables. Materials and Methods: We analyzed LCS prostheses (posterior cruciate ligament [PCL]sacrificing, rotating platform design) in 20 knees of 20 patients. In the current system, there is full contact between the femoral component and the tibial insert from 0° to 30°, and the geometry of the prostheses involves a progressive posterior decrease in the radius of curvature of the femoral condyle, and a decrease in the constraint with flexion between the tibial and femoral components. Thus, 30° is regarded as high constraint, and 75° as low constraint. The clinical characteristics of the patients are summarized in Table 1. AP displacement was measured using a KT-2000 arthrometer (MED metric, San Diego, CA), following a standard protocol, at 30° and 75° flexion. An anterior force of 133 N and a posterior force of 89 N were applied while the patients were under anesthesia, and then after they had regained consciousness. The measurement under anesthesia was performed during TKA surgery on the contralateral side. The average interval between initial TKA and the final measurement was 19 months (range, 6−66 months). We performed measurements in conscious patients within the first week following surgery on the contralateral side of the knee. Three measurements were made and subjected to statistical analysis; intra-subjective errors were < 1 mm. AP joint stability was analyzed by two-way analysis of variance (ANOVA) for repeated measures (triplicate experiments). Results: The mean AP displacements at 30° and 75° in conscious patients were 5.1 mm (range, 1.9−9.3 mm) and 7.0 mm (range, 2.7−11.5 mm), respectively, compared to 6.7 mm (range, 1.7−12.4 mm) and 7.7 mm (range, 3.1−14.0 mm), respectively, in anesthetized patients (Table 2, Fig. 3). AP joint displacement was significantly associated with soft tissue tension (p = 0.026) and constraint (p = 0.001). No interaction was observed between the two variables (p = 0.193; Table 3). Discussion: The effect of anesthesia, which would tend to exclude the contributions of voluntary soft tissue tension, on knee AP joint displacement has primarily been studied as it applies to the repair of the anterior cruciate ligament. In this study, AP in vivo joint displacement was significantly associated with soft tissue tension (p = 0.026), with no interaction between constraints (p = 0.193); thus, AP displacement under anesthesia showed a significantly larger range than in conscious patients, regardless of the different constraints. If the displacement values under anesthesia could be regarded as the same condition as those in an intraoperative one (that is, to exclude the contribution of voluntary soft tissue tension), the results of this study indicate that intraoperative displacement is greater than displacement in conscious patients. Thus, surgeons should be aware that intra-operative AP displacement is larger than the displacement defined as optimal in the awake condition, regardless of the degree of constraint. Moreover, the current study showed that AP joint displacement was significantly associated with constraint (p = 0.001), with no interaction with soft tissue tension (p = 0.193). Surgeons should take into account that higher (or lower) constraints have less (or larger) displacement, regardless of the degree of soft tissue tension. We should not, therefore, determine the proper AP displacement consistently for every PCL-sacrificed TKA, but rather according to the degree of constraint both under anesthesia and while conscious, although this may not hold true for cruciate-retaining TKA designs. Additionally, surgeons should pay careful attention to changes in the conformity of the femoral and tibial couple with flexion, especially in the current prostheses versus those of femoral component design having a single radius. This study has some limitations. First, the interval between measurements of AP displacement varied between 6 and 66 months, which may have affected our results. However, Mizu-uchi et al. [3] reported that AP displacement in joints repaired using the PCL-retaining design without clinical complaints did not change significantly over a 5year period. Additionally, we did not refer to the effect of differences in soft tissue structures, such as in PCL-retaining or -substituting designs, on AP displacement, because this study was intended to analyze only PCL-sacrificed designs. We believe that AP displacement after TKA may also be controlled by the geometry of the prosthesis, soft tissue structures, and their tension, as previously reported by Lafortune et al. [4] in relation to knee kinematics. Moreover, displacement was only measured when there was no axial load, due to the characteristics of the arthrometer that we used. Assessment of displacement under loadbearing conditions may provide a better understanding of the factors influencing clinical performance during activity. In conclusion, this in vivo study demonstrated that AP displacement after TKA was controlled by the degree of constraint and soft tissue tension, without any apparent interaction between these factors. Surgeons should recognize that intraoperative displacement is greater than in conscious patients, regardless of the degree of constraint, and that higher constraints have less displacement, regardless of the degree of soft tissue tension. These results may help surgeons to determine the intra-operative AP displacement required for proper postoperative displacement in each prosthetic design. References: 1. Jones DPG et al. (2006) J Arthroplasty 21: 719 2. Walker PS et al. (1995) Clin Orthop 310: 87 3. Mizu-uchi H et al. (2006) J Arthroplasty 21: 592 4. Lafortune MA et al. (1992) J Biomech 25: 347 Table 1. Patient characteristics.