Abstract
Introduction
Historically, knee implants have been designed using average patient anatomy and despite excellent implant survivorship, patient satisfaction is not consistently achieved. One possibility for this dissatisfaction relates to the individual patient anatomic variability. To reduce this inter-patient variability, recent advances in imaging and manufacturing have allowed for the implementation of patient specific posterior cruciate retaining (PCR) total knee arthroplasty (TKA). These implants are individually made based on a patient's femoral and tibial anatomy determined from a pre-operative CT scan. Although in-vitro studies have demonstrated promising results, there are few studies evaluating these implants in vivo. The objective of this study was to determine the in vivo kinematics for subjects having a customized, individually made(CIM) knee implant or one of several traditional, off-the-shelf (OTS) TKA designs.
Methods
In vivo kinematics were assessed for 108 subjects, 44 having a CIM-PCR-TKA and 64 having one of three standard designs, OTS-PCR-TKA which included symmetric TKA(I), single radius TKA(II) and asymmetric TKA(III) designs. A mobile fluoroscopic system was used to observe subjects during a weight-bearing deep knee bend (DKB), a Chair Rise and Normal Gait. All the subjects were implanted by one of two surgeons and were clinically successful (HSS Score>90). The kinematic comparison between the three designs involved range of motion, femoral translation, axial rotation, and condylar lift-off.
Results
During the DKB, subjects having a CIM-TKA experienced 5.72 (σ=3.03) mm of lateral condyle posterior femoral rollback (PFR) compared to 3.81 (σ=3.08) mm, 1.12 (σ=3.15) mm, and 0.19 (σ=3.53) mm for subjects having the three OTS-TKA designs respectively (Figure 1). For the CIM-TKA 87.5% of subjects demonstrated overall PFR compared to 81%, 72% and 64% of the traditional designs respectively. With Respect to axial rotation, the CIM-TKA demonstrated 4.44° (σ=5.4) of axial rotation compared to 4.59° (σ=3.19), 5.8° (σ=5.58), and 4.41° (σ=5.84) for subjects having the three OTS-TKA designs respectively (Figure 2) and 87.5%, 92%, 76% and 71.4% of the subjects respectively showing normal rotation. These trends were similar with respect to the chair rise activity. During DKB, subjects having a CIM-TKA achieved 107° of weight-bearing knee flexion, while subjects having an OTS-TKA achieved 100°, 97° and 102°, respectively.
Discussion
Subjects with a CIM-TKA experienced posterior motion of their lateral condyle during flexion and an anterior motion during extension, while 19–28% of patients having an OTS-TKA experienced lateral condyle moving in the anterior direction during flexion, paradoxical to the normal knee pattern. In general, the CIM-TKA experienced translation and rotation patterns more similar to the normal knee pattern than the OTS-TKA designs (figure 3). Additionally subjects having an OTS-TKA experienced a higher incidence of anterior sliding and reverse axial rotation. The DKB range of motion was slightly larger for the CIM-TKA and the standard deviation of the lateral condyle motion was smaller for the subjects having a CIM-TKA demonstrating less variability between patients.
Significance
This study found that CIM-CR-TKA Implants demonstrate kinematic patterns more similar to the normal knee, but less in magnitude while experiencing less variability between subjects compared to the OTS-CR-TKA designs.
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