Abstract
Introduction
Cruciate Retaining (CR) and Posterior Stabilizing (PS) are two common types of total knee arthroplasty (TKA) surgeries. The CR approach preserves the posterior cruciate ligament (PCL) while the PS approach sacrifices it. Implant size selection during a TKA surgery is primarily driven by the patient's bone size, but could also be affected by surgery types due to the influence of the PCL. The objective of this study was to investigate the effect of TKA surgery type on implant size selection, based on the clinical database of a well-established commercial implant system.
Methods
A clinical database operated by Exactech, Inc. (Gainesville, FL, USA) was utilized for this study. The database contains TKA patient information of Optetrak® implant recipients from over 30 physicians in the US, UK, and Colombia since 1995. Patient height was used as a control factor for comparison of surgery types, and categorized by every 10 cm (e.g., the “170 cm” category contains patients from 170 to 179 cm). Taking primary TKA only and body heights from 130 cm to 199 cm, a total of 2,677 cases were examined. No statistical difference exists on patients' gender, body weight, or BMI within every height category between the CR and PS groups. The femoral implant size and tibial insert thickness were compared between the two groups.
Results
The implant size generally increases with patient height for both CR and PS groups, except for those under 140 cm (Figure 1). For all height categories, the CR patients received consistently smaller implants than the PS patients (p<0.05). On average, a CR TKA was about 0.67 size smaller than a PS TKA (p<0.001). A 0.67 size corresponds to about 2.7 mm in femoral component's AP dimension. Tibial insert thickness does not vary substantially by patient size (Figure 2). The difference between CR and PS groups was also less significant. On average, the CR group's tibial insert was about 0.5 mm thinner than that of the PS group (10.4 mm vs. 10.9 mm).
Discussion
Flexion-extension gap balancing is a key objective in a TKA surgery. Although it is commonly known in the TKA community that a CR knee tends to have tighter gaps (especially flexion gap) than a PS knee, a quantitative understanding of this subject is lacking. By utilizing a novel statistical method on a single-product database, this study was able to provide a relevant answer to this question. This study found that a CR TKA on average accepted a femoral implant 2.7 mm smaller in AP dimension and a 0.5 mm thinner tibial insert than a PS TKA. Assuming all other factors the same, tibial insert thickness is a reflection of extension gap, while tibial insert thickness plus the AP dimension of the femoral component is a reflection of flexion gap. Thus the gap difference between CR and PS knees is about 0.5 mm at extension and about 3.2 mm at flexion. With statistical evidences, this study indicated that CR surgeons tend to downsize implant (consciously or unconsciously) more often than PS surgeons.
