Abstract
Introduction
Patient specific instrumentation (PSI) is an innovative technology in total knee arthroplasty (TKA). With the use of a preoperative MRI or CT scan, custom guide blocks are individually manufactured for each patient. Contrary to other TKA technologies such as computer-assisted surgery, PSI utilises measured resection technique rather than a primarily ligament balancing technique. This has the potential to negatively affect the operating surgeon's ability to achieve optimal soft tissue balancing, which is especially critical in patients with severe lower extremity malalignment. Despite early research suggesting that PSI is accurate, has a low learning curve, and can reduce operating room time, it remains unclear whether a surgeon using PSI can achieve optimal soft tissue balancing using a measured resection technique. The purpose of this study is to evaluate the efficacy of PSI in patients with severe preoperative limb alignment deformities.
Methods
Fifty PSI total knee arthroplasties were performed on 46 patients (21 male, 25 female) using the Zimmer NexGen Patient Specific Instrumentation system. Each patient included in the study had a minimum preoperative deformity of at least 10° varus or valgus measured on preoperative long leg standing radiographs, Zimmer preoperative software or both. Forty-three of the included knees had a varus deformity and 7 had a valgus deformity. Preoperative mechanical axis alignment measurements were obtained using the PSI preoperative planning software and were manually calculated using preoperative long leg standing radiographs. Postoperative mechanical axis alignment measurements were calculated using plain long leg standing radiographs. The Knee Society Scoring System was used to evaluate clinical and functional outcomes at 1 to 6 months postoperatively.
Results
Average preoperative deformity as calculated with the PSI preoperative planning software and as measured on plain radiographs were 11.5° and 13.3°, respectively. Average postoperative mechanical axis was 3.4° measured from plain radiographs. The average angle between the femoral mechanical axis (FMA) and femoral component, and between the tibial mechanical axis (TMA) and tibial component, was 88.1°. The average difference between the femoral mechanical and anatomic axes was 6.2°. The average discrepancy between medial and lateral joint space on an anterior-posterior standing radiograph was 0.1mm. No patients required soft tissue releases intraoperatively. The American Knee Society criterion showed an aggregate average score of 81.5.
Conclusion
Patient specific instrumentation (PSI) is an innovative technology in TKA utilising a measured resection technique. It has not been previously established whether or not this technology is effective at restoring optimal soft tissue balance in TKA, particularly in individuals with severe preoperative alignment deformities. This study demonstrates that PSI is capable of producing favourable radiographic and clinical outcomes in a subset of patients with at least 10° of malalignment pre-operatively. Given the American Knee Society scores as well as the radiographic findings of an average mechanical axis of 3.4° post-operatively, we believe that this technique can achieve comparable results to techniques using intraoperative soft tissue balancing. We believe PSI is an accurate and effective tool for use in patients with severe preoperative angular deformities of the knee.
