Abstract
Background
Despite the success of total knee arthroplasty (TKA) restoration of normal function is often not achieved. Soft tissue balance is a major factor for poor outcomes including malalignment, instability, excessive wear, and subluxation. Computer navigation and robotic-assisted systems have increased the accuracy of prosthetic component placement. On the other hand, soft tissue balancing remains an art, relying on a qualitative feel for the balance of the knee, and is developed over years of practice
Several instruments are available to assist surgeons in estimating soft tissue balance. However, mechanical devices only measure the joint space in full extension and at 90° flexion. Further, because of lack of comprehensive characterization of the ligament balance of healthy knees, surgeons do not have quantitative guidelines relating the stability of an implanted to that of the normal knee. This study measures the ligament balance of normal knees and tests the accuracy of two mechanical distraction instruments and an electronic distraction instrument.
Methods
Cadaver specimens were mounted on a custom knee rig and on the AMTI VIVO which replicated passive kinematics. A six-axis load cell and an infrared tracking system was used to document the kinematics and the forces acting on the knee.
Dynamic knee laxity was measured under 10Nm of varus/valgus moment, 10Nm of axial rotational moment, and 200N of AP shear. Measurements were repeated after transecting the anterior cruciate ligament, after TKA, and after transecting the posterior cruciate ligament.
The accuracy and reproducibility of two mechanical and one electronic distraction device was measured.
Results
The maximum passive varus laxity measured over the range of flexion was 6.4°(±2.0) and maximum passive valgus laxity was 2.6°(±0.7), (p < 0.05). The maximum passive rotational laxity measured was 9.0°(±0.57) for internal and 14.1°(±1.6) for external rotation (p < 0.05). Average stiffness of the knee (Nm/deg) was 1.7 (varus), 2.4 (valgus), 0.8 (internal rotation), and 0.5 (external rotation). The difference in tibiofemoral gap between flexion and extension was 2.9mm (±1.6).
The stiffness of the mechanical and electronic distractors was very linear over a distraction range of 0 to 6mm. At forces ranging from 40N to 120N, the accuracy and repeatability of the mechanical distractors was within 1mm, and that of the dynamic electronic distractor was 0.2mm. The electronic distractor measured the varus of the tibial cut and the distal femoral cut within 0.5°, and the rotation of the posterior femoral cut within 0.7° of surgical navigation measurements.
Conclusions
The dynamic electronic distraction device was significantly more accurate than mechanical instruments and measured knee balance over the entire range of flexion. The stiffness of the normal knee was distinctly different in varus and valgus. The standard recommendation for equal medial and lateral gaps under distraction may have to be revisited. Combining implant design improvements with sophisticated balancing instruments is likely to make a significant impact on improving function after total knee arthroplasty.
