Jean-Yves Jenny, MD
Hôpitaux Universitaires de Strasbourg
Michael B. Cross, MD
Hospital for Special Surgery
Abnormal knee kinematics have been found in fluoroscopy studies after total knee arthroplasty (TKA)1, which may lead to suboptimal clinical outcomes2. For cruciate-retaining (CR) TKA, the posterior tibial slope (PTS) of the reconstructed proximal tibia has been theorized to play a significant role in restoring normal knee kinematics as it directly affects the tension of the posterior cruciate ligament (PCL)3,4. Thus, the need for a deeper understanding of the impact of PTS has driven current research on the postoperative kinematics of the CR knee, which traditionally has been carried out by conventional cadaveric testing. However the reproducibility of such cadaveric testing may not be acceptable, as repetitive changes of the PTS due to removal of trial implants from cadaveric bone may damage the soft tissues in the knee. Although some soft tissue preserving methods have been proposed for the adjustment of the PTS, such as performing an anterior opening wedge osteotomy and filling the gap with bone cement4, these proposed methods may lead to inaccuracy in PTS, as results can be affected by variability when performing the osteotomy and/or cement curing.
In this present study, a novel, soft-tissue preserving method for measuring the effects of PTS on the knee kinematics was developed and verified. A preliminary analysis was performed on one cadaveric knee to assess the impact of PTS on the kinematics of a CR TKA.
Materials and Methods
A cemented CR TKA (Optetrak Logic® CR, Exactech, Gainesville, FL) was performed using a computer-assisted surgical guidance system (ExactechGPS®, Blue-Ortho, Grenoble, FR) on one fresh-frozen non-arthritic cadaveric knee with an intact PCL. The tibial baseplate was custom designed with a mechanism to precisely and easily modify the PTS without the need to repeatedly remove and assemble tibial inserts of varying posterior slopes (as offered by the Optetrak Logic CR system) (Fig. 1). Postoperative tibiofemoral internal/external (I/E) rotation, tibiofemoral anteroposterior (AP) translation, and hip-knee-ankle angles (HKA) were evaluated by performing a full passive range of motion (ROM) (flexion angle from 0° to 120°) three separate times at each of the five experimental PTSs in the following order: 10°, 7°, 4°, 1°, and back to 10° at the end of the testing. The novel test method with the custom designed tibial baseplate was verified by assessing any potential damage of the PCL or other soft tissues by comparing the kinematic data of the initial and the last experimental conditions at 10° PTS. Statistical comparisons were performed at ~0° (3°), 30°, 60°, 90° and 120° flexion angles. The kinematics were then also compared across PTSs for the specific knee investigated (the first 10°, 7°, 4°, and 1°). Statistical significance was defined as p < 0.05.
Figure 1: A custom designed tibial baseplate for the test. Turning the anterior screw (1) results in modification of the posterior tibial slope (2).
Test method verification
Similar knee kinematics were observed between the two sets of data acquisitions at 10° PTS (Fig. 2), with the tibiofemoral AP translation being nearly identical. No significant differences were found between the two sets of data at the sampled flexion angles, except for an arguably clinically negligible difference in the tibiofemoral AP translation at 30° flexion (p = 0.04, difference in means <1mm).
Impact of PTS on knee kinematics
All four PTSs generally had similar kinematic patterns across all flexion angles. However, clinically significant changes in the normal tibiofemoral I/E rotational kinematics were found for PTSs of 1° and 4°, while PTSs of 7° and 10° led to I/E rotational kinematics close to the normal knee (Fig. 3A). Similar AP kinematics were found between the native knee and all four PTSs (Fig. 3B). All PTSs closely follow the same pattern in HKA, which was different than the native knee (Fig. 3C). However, the differences in HKA may not be clinically significant (less than 2°-3°).
Figure 2: A) tibiofemoral I/E rotation, B) HKA, and C) tibiofemoral AP translation as a function of the flexion, compared between the initial and the last acquisitions at 10° PTS.
Figure 3. Comparison of A) tibiofemoral I/E rotation, B) tibiofemoral AP translation, and C) HKA between the native knee and the component implanted at different PTSs.
This pilot study demonstrated that the novel test method developed does not significantly disrupt the soft tissue environment of the knee. Previous evaluations of the effect of the PTS on passive knee kinematics often manipulate the PTS by re-cutting the proximal tibia and/or frequently exchanging the tibial insert, which has been shown to damage the PCL and/or stretch the soft tissue envelope5. As a result, those studies may have inherent flaws that prevent meaningful data collection. The novel tibial baseplate designed for this study adjusts the PTS without re-cutting the tibia and removing the components, therefore offers a reliable test method that respects the soft tissue envelope in the knee.
The preliminary results presented here showed that PTS can impact the kinematics in the knee. For the specific knee and implant design investigated, the impact of the PTS was explicit in the tibiofemoral I/E rotation and AP translation, which may be related to the strain in the PCL. In contrast, PTS impacted the HKA to a lesser degree.
Based on this pilot study, the authors recommend the proposed test method for future investigations on the effect of PTS on the postoperative knee kinematics.
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