Abstract
Summary:
Smaller increments in the antero-posterior dimensions of femoral components allows significant improvements in balancing of the knee after TKA with restoration of more normal soft-tissue stability.
Introduction:
The soft-tissue stability of the knee after TKA is often compromised by the fact that only a finite set of implantable component sizes is available to match bony anatomy. While this could be overcome with custom components, a more practical solution is a set of femoral components with smaller increments in the antero-posterior (AP) dimension. However, this results in a larger assortment of sizes of both implants and trial components. This study was performed to determine whether smaller increments in the AP sizing of knee prostheses would lead to real benefits in restoration of normal knee function and stability after TKA.
Methods:
Specimen-specific computer models of 5 cadaveric knees were created through reconstruction of computer tomography scans and co-registered magnetic resonance images. Modeled elements simulating all mechanically significant soft-tissue structures were incorporated in each model and calibrated through multi-axial robotic testing of each cadaveric specimen. The usage of the specimens was approved by the local ethics commission.
A contemporary design of posterior-stabilized TKA was virtually implanted in each knee, based on in-vitro reference implantations performed by an orthopedic surgeon. The effect of implant sizing was evaluated by evaluating the change in laxity of each knee after varying the AP size of the femoral component by +/− 2 mm, and then by +/− 1 mm. This corresponded to the change between sizes of a conventional implant system with 4 mm increments vs. an alternative sizing scheme with increments of only 2 mm.
For each implant selection, the laxity of the TKA at 0°, 30°, 60°, 90° and 120° of flexion was predicted in response to varus-valgus (v/v) moments of +/− 10 Nm, internal-external (int/ext) torques of +/− 6 Nm and AP shear loads of +/− 100 N under a compressive load of 44N.
Results:
When the femoral component was down-sized by 2 mm (4 mm increment system), knee laxity at 90° of flexion increased by 3.1° in v/v (range: 2.7°–3.5°), 7.1° in int/ext rotation (range: 6.4°–8.6°) and 3.7 mm in AP translation (range: 2.1–6.3 mm). Up-sizing caused each component of knee laxity to decrease by a similar amount (Figure 1). When the increments in femoral component sizing were reduced to 2 mm, each component of knee laxity was reduced by approximately 50% (v/v: 1.5°; int/ext rotation: 3.6°, and 2.0 mm AP translation; all p < 0.05). In extension, no relevant changes in knee laxity were detected.
Discussion and Conclusion:
This study demonstrates that the use of femoral components with 2 mm AP increments allows much closer approximation of the normal laxity characteristics of the knee. This may enable surgeons to improve ligament balancing and the creation of a knee that feels more natural after joint replacement.
