Total knee arthroplasty (TKA) is an extremely successful operation. Technical success depends on accurate axial alignment, proper soft tissue balance, and equalizing flexion and extension gaps. Traditional instrumentation relies on intramedullary (IM) femoral instruments and either IM or extramedullary (EM) tibial instruments to obtain proper axial alignment. Failure to obtain proper axial alignment may lead to premature loosening7 or decreased survival.9
Traditional instrumentation is essentially similar among manufacturers and has remained relatively unchanged over the last 25 years, showing the utility and consistency of the method. However, sometimes it is not possible or not appropriate to obtain proper axial alignment with IM instruments. Intramedullary instruments cannot be used in patients with previous trauma and substantial residual bony deformity, retained hardware that cannot be removed, and may increase risks for patients with a history of diaphyseal osteomyelitis, severe cardiopulmonary disease, or a patent foramen ovale.2 These problems can be circumvented with extramedullary (EM) jigs on the tibial side, but EM instrumentation is cumbersome on the femoral side, which requires radiographically identifying the femoral head and a freehand technique of pinning the distal femoral resection guide.
An alternative to this approach is using computer- assisted surgery (CAS) to obtain proper femoral alignment. Proper alignment has been defined as within ± 3° of the mechanical axis7 or between 0° and 4° of valgus.9 Computer-assisted surgery can be used to assist the surgeon with proper component alignment and soft tissue balancing in TKA. Although controversy exists with regard to operative time and cost, multiple prospective studies have shown improvement in component positioning when using CAS compared with traditional methods.1,4,5,10,11 We reasoned if CAS were successful in obtaining improved alignment in routine primary TKA, then it may be useful in situations where IM femoral instrumentation is contraindicated.
We asked whether CAS could properly align a TKA in patients with severe bony deformities or when traditional instrumentation was not possible or appropriate.
MATERIALS AND METHODS
We retrospectively reviewed databases from three institutions to identify patients who underwent CAS for proper implant alignment when femoral IM instrumentation was not possible or not appropriate then ascertained final alignment. We defined the following inclusion criteria: (1) severe posttraumatic femoral deformity when one is unable to pass an IM guide to accurately make a distal femoral cut (Fig 1); (2) retained hardware that would be difficult or inadvisable to remove (Fig 2); (3) femoral canals with a history of osteomyelitis; and (4) patients with severe cardiopulmonary disease or a known patent foramen ovale who may be at risk for embolic showers because of femoral IM instrumentation. Patients treated using CAS for extraarticular tibial posttraumatic deformity, tibial osteomyelitis, or retained tibial hardware were excluded. While CAS can be helpful in these situations, we felt it was not essential. Extramedullary alignment devices can usually provide accurate tibial alignment in these situations. Sixteen patients (18 knees) met the inclusion criteria. Eight patients had isolated femoral deformities, one patient had retained hardware and a femoral deformity, and two patients had retained hardware. Two patients had a history of femoral osteomyelitis, one of which was accompanied by a femoral malunion. Three patients (4 knees) had severe cardio- pulmonary disease or a patent foramen ovale. These elderly patients, age 63 (patient 6), 79 (patient 8), and 85 (patient 12) had ASA Classifications of 3, 2, and 4 respectively.
Five surgeons with varying degrees of CAS experience contributed patients to this study. Their experience ranged from routine use to selective use to occasional use. All procedures were performed using a computed tomography (CT) free optical tracking CAS system (Ci System, Brain Lab, DePuy J & J Orthopaedics, Warsaw, IN).
Each patient had a detailed radiographic analysis performed postoperatively. We obtained hip, knee, ankle radiographs standardized for proper rotation. Coronal alignment was determined by measuring the mechanical axis of the femur, the mechanical axis of the tibia, the overall mechanical axis of the limb using the radiographs. The overall mechanical axis of the limb was considered acceptable if the line from the middle of the femoral head to the middle of the talus fell within the box of the posterior- stabilized implant.7 A substantial difference in the incidence of loosening has been noted in implants where a line from the center of the femoral head falls outside the middle third of the implant.7 All radiographs were measured by one of the authors (TKF).
The sagittal femoral angle was measured but not believed valid when performed on standard lateral radiographs. In order to appropriately measure this angle in a sagittally deformed femur, a properly rotated full length lateral radiograph similar to a coronal hip-knee-ankle view must be taken. This radiograph is difficult to obtain and has not been validated for accuracy. The sagittal tibial component angle was determined on a properly rotated lateral radiograph. Deviation from normal concerning each of these measurements was recorded for each patient.
We calculated the average deviation and range for each measurement as well as a 95% confidence interval (CI) and standard deviation (SD).
We successfully used CAS in 17 knees. In one morbidly obese (258 lbs, BMI 43) patient with severe pulmonary disease, we were unable to stabilize the pelvis in order to accurately register the hip. Therefore, CAS was abandoned and the surgery was successfully completed without sequelae with conventional instrumentation and careful femoral lavage.
The overall mechanical axis of the limb fell within the posterior stabilized box in 16 of the 17 knees treated using CAS (Table 1). One patient with a substantial posttraumatic varus extension biplane deformity of the femur had an overall mechanical axis that fell just outside the posterior-stabilized box on the hip-knee-ankle view. This outlier was secondary to a femoral mechanical axis of 4° of varus.
The femoral mechanical axis was within ± 3° of neutral in 16 of 17 knees treated using CAS. The average femoral mechanical axis was 89.9 (SD, 1.3; 95% CI, ± 0.6). The femoral mechanical axis was neutral in 14 patients. Two patients had a femoral mechanical axis in 1° and 4° of varus and one patient was in 3° of valgus.
The tibial mechanical axis was within 1° of neutral in all patients. The average tibial mechanical axis was 89.9 (SD, 0.60; 95% CI, ± 0.3). The tibial mechanical axis was neutral in 11 of 17 patients. The remaining six patients had a tibial mechanical axis that deviated slightly from neutral. Four patients were in 1° of varus while two patients were in 1° of valgus.
The sagittal tibial angle was within ± 3° in all 17 patients treated using CAS. The average sagittal tibial angle was 89.9 (SD, 1.8; 95% CI, ± 0.65). The sagittal tibial angle was neutral in eight patients. Four patients had 1° of posterior slope. Two patients had 2° of posterior slope. Three patients had reverse slope of 1°, 2°, and 3° respectively.
The success of TKA depends on proper axial alignment, soft tissue balancing, and equalizing the flexion and extension gaps. We used CAS to facilitate proper implant alignment. The proof of any new technology is how well it performs in a worst case scenario. We studied if CAS could properly align a TKA with severe bony deformity or when IM instrumentation was not possible or not appropriate.
The limitations include a relatively small data set (17 knees) from three centers. The limited accuracy in measuring angles on plain radiographs also accounts for small errors in alignment. While the long views we used to measure coronal alignment were more precise than short leg radiographs,3 they were still subject to inherent measurement errors. We attempted to standardize our postoperative radiographs to minimize such errors. Another limitation was the lack of a control group in which a freehand technique using intraoperative radiographs and EM jigs were used when femoral IM instrumentation was contra- indicated. We attempted to define a historical control group where this technique was used, but we were unable to extract these patients from our database. Despite these limitations, we believe CAS was sufficiently accurate in achieving proper alignment when femoral IM instrumentation was contraindicated (Fig 3A-C).
Our data were comparable to those in studies documenting the superiority of CAS compared with traditional IM instrumentation.1,4,5,8,10,11 (Table 2). These studies demonstrated a marked improvement in alignment parameters when comparing traditional manual navigation with computer-assisted navigation techniques.1,4,5,10,11
One would expect the results for the seven patients in our series treated with CAS for retained hardware, severe cardiopulmonary disease, or a history of osteomyelitis would not differ from the reported literature because there was no major associated bony deformity. However, all but one of our patients with posttraumatic angular bony deformity had an acceptable overall mechanical axis postoperatively.
While our study and the multiple comparative studies in the literature have demonstrated the accuracy of CAS, controversy still exists concerning whether malalignment contributes to implant failure.4 Some authors consider failure to obtain proper alignment a risk factor for loosening or decreased survivorship,7,9, while others do not.6,12 While minor degrees of malalignment (1°-3°) are clearly well tolerated and rarely result in revision, the goal of any alignment system is to prevent outliers (errors > 3°). While not a source of early failure, these outliers may have implications with regard to polyethylene wear or aseptic loosening at extended followup.
Controversy remains concerning whether the additional operating room time and added expense will have a positive risk-benefit ratio for routine TKA. In this era of declining reimbursement, the cost-effectiveness of CAS remains unsettled. We found CAS was a valuable adjunct to obtaining proper alignment in difficult situations.
Proper alignment is difficult to obtain when bony deformities, retained hardware, a history of osteomyelitis, or severe cardiopulmonary disease prevents the use of traditional femoral instrumentation. Computer-assisted navigation was effective in treating patients when traditional instrumentation was not applicable.
Because there is a learning curve for any new technology, it is important for the operative surgeon to become proficient with computer-assisted primary knee replacement before attempting its use in these complex deformities. Early adopters of computer-assisted knee replacement cite alignment improvements over conventional techniques as ethical justification for the added expense and operative time incurred. While intuitively improved alignment should improve results, this remains to be demonstrated in any long-term outcome study. Those surgeons who are waiting on such long-term studies before deciding to use computer-assisted surgery routinely in their practice should use alternative techniques of obtaining proper alignment in these complex deformities or consider referring these patients to centers who use CAS routinely.
Special thanks to Jean Dehner for help in preparation of this manuscript.
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