Progression of varus deformity in osteoarthritic knees induces anterior paradoxical motion of the femur during early knee flexion.

PURPOSE: The purpose of this study was to investigate the position of the femur relative to the tibia throughout range of motion in the osteoarthritic knee to evaluate knee kinematics and assess its relationship with the degree of varus deformity. METHODS: In this study, 116 preoperative knees with varus deformity were evaluated using a navigation system. The internal-external, anteroposterior, and mediolateral positions of the femur relative to the tibia were measured at maximum extension, 15°, 30°, 45°, 60°, 90°, 105°, and 120°, and maximum flexion angles. From these parameters, two-dimensional translation of the surgical epicondylar axis was projected onto the tibial axial plane, and the femoral movement was evaluated relative to the tibia. In addition, the knees were retrospectively classified into three groups according to their degrees of preoperative hip-knee-ankle angle: mild (< 10°), moderate (10°-20°), and severe (> 20°). Then, the differences in each parameter between these groups were investigated. The Steel-Dwass test was performed to identify the difference among three groups. Statistical significance was set at p values < 0.05. RESULTS: There was a significant difference in the anteroposterior position of the femur relative to the tibia among the three groups, especially from extension to early flexion (p < 0.05). The anteroposterior position at knee extension deviated posteriorly according to the progression of varus deformity. Rotational and mediolateral translation were not significantly different among the groups. Normal knee kinematics were diminished in almost all cases in each group. In addition, anterior paradoxical motion of the femur during early knee flexion was observed in 45.6% (n = 26), 57.1% (n = 28), and 80.0% (n = 8) of cases in the mild, moderate, and severe groups, respectively. The anteroposterior position of the femur relative to the tibia at knee extension was significantly more posterior in patients with than in those without anterior paradoxical motion (p < 0.0001). CONCLUSION: The anteroposterior position of the femur relative to the tibia changed according to the progression of varus deformity in osteoarthritic knees, especially from knee extension to early flexion. Posterior deviation of the femur at knee extension induced its anteroposterior movement relative to the tibia, resulting in anterior paradoxical motion. LEVEL OF EVIDENCE: III.

Intraoperative posterior movement of the tibia at 90° of flexion predicts worse postoperative flexion angles in cruciate-substituting total knee arthroplasty.

PURPOSE: To investigate the correlation between intraoperative tibiofemoral anteroposterior changes at 90° of flexion and postoperative maximum flexion angles in navigated cruciate-substituting TKA. The hypothesis of this study was that intraoperative tibiofemoral anteroposterior changes at 90° of flexion indirectly reflect posterior cruciate ligament (PCL) function and associate with postoperative maximum flexion angles. METHODS: Fifty-five consecutive patients with varus osteoarthritis treated with primary TKA were retrospectively analysed. All patients received the same type of implant, placed with an image-free navigation system. The PCL was retained, and cruciate-substituting inserts were used in all cases. The mean follow-up was 44 ± 8 months. The preoperative and postoperative kinematics were measured intraoperatively with a navigation system, and the preoperative and postoperative tibiofemoral anteroposterior positions at 90° of flexion were determined. The correlation between intraoperative anteroposterior position changes and postoperative maximum flexion angles was investigated. The correlation between the change of anteroposterior position and tibiofemoral rotational angles was also assessed. RESULTS: The intraoperative anteroposterior position change was -1.7 ± 3.4 mm (a positive value indicates tibial posterior shift). Flexion angle improvement was negatively correlated with intraoperative change of tibiofemoral anteroposterior position (R2 = 0.17, p < 0.005). Postoperative maximum flexion angles were also negatively correlated with intraoperative change of tibiofemoral anteroposterior position (R2 = 0.09, p < 0.05). The postoperative amount of tibial internal rotation was positively correlated with the preoperative amount (R2 = 0.60, p < 0.0001); however, the intraoperative anteroposterior position change was not correlated with the postoperative amount of tibial internal rotation (n.s.). CONCLUSION: A navigation system may be able to indirectly evaluate PCL function and predict the postoperative flexion angles in cruciate-substituting TKA. Intraoperative posterior movement of the tibia at 90° of flexion predicts worse postoperative flexion angles in cruciate-substituting TKA. LEVEL OF EVIDENCE: Level 3, retrospective comparative study.

Anterior cruciate ligament-deficient knee induces a posterior location of the femur in the medial compartment during squatting.

Although an anterior cruciate ligament (ACL) deficiency induces knee osteoarthritis, particularly in the medial compartment, the kinematics remains partially unclear. This study investigated the in vivo knee kinematics of ACL-deficient and normal knees by comparing them during squatting. This prospective comparative study included 17 ACL-deficient knees and 20 normal knees. The kinematics was investigated under fluoroscopy using a two- to three-dimensional registration technique. The anteroposterior (AP) translation of the medial and lateral sides of the femur, axial rotation of the femur relative to the tibia, and kinematic pathways were evaluated and compared. At first, the medial AP position of the femur translated anteriorly from 0° to midflexion, afterward it translated posteriorly in both ACL-deficient and normal knees. However, the medial AP position of the femur in the ACL-deficient knees was located significantly posteriorly compared with normal knees at 0-110° flexion. The lateral AP position of the femur translated posteriorly from 0° to 150° flexion in both ACL-deficient and normal knees. The lateral AP position of the femur in the ACL-deficient knees was located significantly posteriorly compared with that in normal knees at 0-10° flexion. The femur showed external rotation from 0° to 150° flexion in both ACL-deficient and normal knees. A medial pivot motion and subsequent bicondylar rollback were observed in both knees in the kinematic pathway. In conclusion, the AP position of the femur relative to the tibia was altered due to ACL deficiency, particularly in the medial compartment.

Postoperative femoral anteroposterior position while standing correlates with the posterior tibial slope and posterior femoral condylar offset in medial pivot total knee arthroplasty.

BACKGROUND: After total knee arthroplasty (TKA), the femur tends to be located posteriorly under weight-bearing (WB) conditions, and a paradoxical femoral anterior motion occurs, leading to joint instability. OBJECTIVE: We aimed to clarify factors which affect the postoperative femoral anteroposterior position relative to the tibia under WB conditions (A-P position) in medial pivot (MP) TKA. METHODS: Among 126 knees (81 women) with primary TKA using MP prosthesis for varus osteoarthritic knees, 70 knees had cruciate-retaining inserts (CR) with the reduced conformity and 56 knees had cruciate-substituting inserts (CS) with the full conformity. Using the three-dimensional (3D) assessment system, the associations between the A-P position and the factors including pre- and postoperative lower extremity alignment, component positions, and posterior femoral condylar offset (PCO), were assessed regarding the type of inserts. RESULTS: Significant correlations were seen between the A-P position and posterior tibial slope (PTS), medial PCO, and lateral PCO. Regarding the difference between the two inserts, the PTS, medial PCO, and lateral PCO significantly correlated with the A-P position in the CR, but only the PTS correlated in the CS. CONCLUSIONS: The increased PTS and decreased PCO were the dominant factors for the A-P position in MP TKA.