Intra-operative characterization of knee laxity during total knee arthroplasty

INTRODUCTION: Achieving proper knee laxity is an important yet challenging part of total knee arthroplasty (TKA) [1]. Knee laxity during TKA is usually assessed as a surgeon manually manipulates the knee and decides if the joint has acceptable laxity and stiffness. This subjective method does not objectively quantify laxity, has unknown repeatability, and is difficult to teach to younger surgeons. Recently, navigation systems [2] and instrumented sensors [3] have provided objective measurements of passive knee kinematics and tibiofemoral contact, respectively, but do not biomechanically characterize knee movements due to a given load. Ultimately, precisely how a TKA changes the laxity of a knee with osteoarthritis (OA) remains unknown. We used a custom navigation system and intra-operative device to 1) objectively characterize changes in knee laxity due to TKA, 2) objectively determine whether TKA resulted in a balanced knee and 3) compare measurements of TKA laxity against values of knee laxity from non-osteoarthritic knees. METHODS: This study followed a protocol that was approved by the Institutional Review Board at The Ohio State University. Increased surgical time associated with making measurements was limited to 20 minutes. A cohort of 35 patients participated in this study with 1 bilateral patient for a total of 36 knees, 10 males and 25 females, with an average age of 60.4±8.2 years, all with a pre-operative varus deformity. A series of tests measuring knee laxity in the anterior-posterior (AP) and varus-valgus (VV) directions were then conducted before and after total knee arthroplasty. Two experienced surgeons exposed the knees using a medial parapatellar approach and then attached optical trackers from our custom surgical navigation system and knee laxity device [4]. Force- displacement data were recorded with the knee in full extension by measuring the forces applied to the lower limb by the surgeon via an instrumented handle as it was moved in the sagittal and frontal planes independently, as we have reported previously [4,5]. The surgeons then performed a total knee arthroplasty with a posterior stabilizing implant (Zimmer NexGen LPS flex knee, Warsaw, IN), and the measurements were repeated after cementing in the final components. Surgeons were blinded to the measurements while they were taken inside of the operating room. The knees' mediolateral balance was calculated by the amount of varus or valgus excursion that occurred when ±10 Nm was applied to the knee with respect to the angle of the knee with 0 Nm applied moment. A general linear model ANOVA was used to test for significance of surgeon and OA vs. TKA knee condition on varus-valgus and anterior-posterior laxity. Paired t-tests were used to test for differences in mediolateral balance. A two sample t-test was used to compare both OA and TKA knee stability data with knee laxity data from a cohort of 46 native cadaveric knees. Each previous cadaveric test used our custom surgical navigation system and device and followed similar protocol to the present study. RESULTS: Performing a TKA resulted, on average, in a “looser” knee in the varus-valgus direction (5.7° ± 3.0°) compared to OA knees (4.7° ± 2.9°) (p=0.02, Table 1). On average, AP laxity did not change (OA= 4.4 ± 4.6 mm, TKA= 5.7 ± 5.4mm,) (p=0.25, Table 1). However, there was great variability in these results. 20 knees were “looser” in the varus-valgus direction, while 16 were “tighter”. Similarly, 17 knees were “looser” in the AP direction, while 16 were “tighter” (3 knees had measurement errors in the AP direction and were excluded). The manner in which this TKA laxity was achieved differed by surgeon in the varus-valgus direction, as indicated by the significant surgeon x OA/TKA interaction effect (Table 1). Surgeon 1 saw OA knees with 3.4°±2.2° of varus-valgus laxity and left the knees looser (6.4°±3.3°). Conversely, surgeon 2's cohort initially had 5.7°±3.0° of varus-valgus laxity, and he did not substantially alter it after TKA (5.2°±2.9°). The knees recorded in the operating room were also balanced post-operatively on average, with no differences seen between varus and valgus laxity at ±10Nm (p = 0.965) or at the maximum applied varus-valgus moment (p = 0.973). Both OA knees (varus-valgus p<.001 and anterior-posterior p=0.047 Table 2) and TKA knees (varus-valgus p<.001 and anterior-posterior p<.007, Table 2) measured in the operating room were “looser” than the cohort of native cadaveric knees. DISCUSSION: This study represents the first objective characterization of intra-operative knee stability in response to a measured applied load, and is the first to characterize each knee both pre and post implantation of the prosthetic components. We previously demonstrated that a TKA results in greater knee laxity both in cadavers (in response to an applied load [4]) and in the operating room (the passive range of motion [6]). These results confirm that there is a difference between the OA and TKA knees in the varus-valgus direction, with the TKA knees being looser. However, in the anterior-posterior direction, TKA knees remained unchanged compared to the OA knees. This study identifies an interesting question as to how post-operative laxity is established with TKA. Even though, on average, the varus-valgus laxity for the TKA knees was similar between the 2 surgeons and was looser than the OA knees, surgeon 1 tended to “loosen” a cohort of patients with “tighter” knees, while surgeon 2 did not appreciably alter varus-valgus laxity. As both surgeons loosened and tightened knees within their respective cohorts, it remains unknown whether our results are due to the desire of both surgeons to leave the knees in a similar state post-operatively or whether our results would be changed if the surgeons were presented with a different cohort of patients and they continued their general approach of either loosening or not altering laxity. Differences between TKA knees and the cadaver cohort show that non-OA (“normal”) knee laxity is also not achieved. Differences seen between OA knees and the cadaver cohort may be due to differences between healthy and osteoarthritic knees [7], as the cadaveric cohort had OA ranging from none to mild. It is unknown if TKA should attempt to restore the pre-operative condition of the joint, to biomechanical balance, or to non-OA knee laxity conditions. Our results indicate that the joint was restored to biomechanical balance, however the pre-operative laxity conditions of the joint may (AP laxity) or may not (VV laxity) be retained following TKA.