Both groups had similar results for all clinical parameters except the findings for ligament balancing in 30° and 90° flexion. An increased (p < 0.001) Insall knee score was found with time in both study groups but the increases were similar (Table 4) (Fig 3). The postoperative ROM in both groups also was similar at the 3-month and at the 2-year followups. Both groups had an increase (p ≤ 0.002) in ROM between the 3-month and the 2-year followups. The improvement of ROM was similar in the CT-based (13%) group compared with the imageless group (7%). Patients' satisfaction was rated as very good or good in 14 of 21 patients in the CT-based group and in 19 of 21 patients of the imageless group (Fig 4). One patient in each study group had anterior knee pain. Navigation-related perioperative and postoperative clinical complications could not be detected. During clinical examination, two of five ligament-balancing parameters showed a better soft tissue situation in patients in the imageless group (Table 4). Our examination of ligament balancing in full extension was similar in the groups. In 30° flexion, we detected better (p = 0.004) medial and lateral stability in the imageless group. The anterior drawer test showed better (p = 0.035) stability in the imageless navigation group. Patients in both groups answered the question regarding instability in a similar manner. Two patients needed help from the examiner to do the step test: one in the CT-based group because of insufficient ROM of 85° and one in the imageless group because of pain.
Using the CT-based navigation module is approximately 42 minutes more time consuming preoperatively, requires more organization, and is more expensive. Performing the preoperative CT scan took 20 to 30 minutes. The mean time for the preoperative planning procedure for CT-based navigation was 17 ± 3 minutes (range, 13-21 minutes). The durations of surgery for the CT-based and imageless navigation techniques were similar (100 ± 17 minutes and 107 ± 19 minutes, respectively).
Several clinical studies showed there was an increased incidence of aseptic loosening and long-term failure of TKAs in patients with a reconstructed mechanical axis after TKAs exceeding ± 3° inaccuracy in the coronal plane.2,13,15,26 A range of 3° inaccuracy (varus or valgus) of an implanted mobile-bearing tibial component was confirmed in a simulated study18 as maximum in vivo tolerability. A rate of 24% for aseptic loosening15 was reported in patients with an alignment in the coronal plane exceeding ± 3°. In comparison, the loosening rate was 3% when the deviation from a neutral axis was within ± 3°. A 90% 10-year survival rate was reported after TKAs, with a tolerable range of 4° for varus/valgus deviation in the coronal plane.25 The survival rate decreased to 73% in patients with varus deformities and 71% in patients with valgus deformities when the mechanical axis inaccuracy exceeded ± 4°. Failure to achieve mechanical alignment of the limb within 3° varus/valgus using conventional implantation instruments during TKAs have been reported in as much as 26% of patients.19,24,33
Our study has several limitations. The first is the absence of randomization because at the beginning of the study, only the CT-based navigation module was available for us. However, all preoperative measured parameters were similar between the two groups (Table 2). Therefore, the preconditions in the two patient groups are comparable. The surgeon was well trained in using the two different modules of the navigation system (cadaver tests and 20 initial patients) before patient recruitment. Therefore, the conclusions might not be applicable to someone less familiar with the techniques. The postoperative radio-graphic measurements and clinical examinations were performed once by one orthopaedic surgeon (CS) not involved in the surgical procedures and who was blinded to the results. Although one observer eliminates variability, one observer may introduce systematic bias. Our study also does not allow for conclusions regarding tibial and femoral component rotational position (postoperative CT was not possible within the agreement of the ethical committee). However, during the preoperative planning procedure in the CT-based navigation group, there were variations in the definition of the transepicondylar axis of as much as 4°. Therefore, there is variability in the CT-assisted definition of the surgical transepicondylar line, indicating it is not an absolute clearly definable anatomic landmark. This finding was confirmed in independent studies that showed unreliable precision of transepicondylar axis definition.1,11,16,17 The intraoperative definition of the femoral component rotation is best based on a combination of all available landmarks (posterior condylar line, surgical transepicondylar axis, AP line, and the actual medial and lateral ligament balancing environment in 90° flexion).
Our data for limb alignment in the coronal plane, tibial component positioning in the coronal and sagittal planes, and coronal femoral component position are comparable to those in previous studies5,9,12,20,22,23,28,30,31,35 using CT-based or imageless navigation systems (Table 5). The findings for postoperative varus and valgus inaccuracies were confirmed in an experimental cadaver study21 using a CT-based navigation system. The cumulative error of alignment showed a high rate of optimal component position in all measured planes in both groups. We found no differences using the CT-based or imageless navigation module. These findings were in agreement with the results of another comparative study.3,4 In addition, we could not identify alignment differences in either study group between the two followups. The accuracies of the mechanical alignment of the limb reported in the literature using conventional intramedullary or extramedullary instruments without a navigation system are less than in both of our study groups.6,7,19,24,33,34
The femoral component should be implanted according to the distal femoral anterior cortex, avoiding cortical notching. This requirement also can be achieved using conventional implantation instruments (Fig 5A). The latter was confirmed by a prospective randomized study9 that established no difference for the femoral component orientation using a navigation system or a conventional jig-based surgical technique. With the exception of the CT-controlled studies,8,9,31 a scientific statement of the orientation of the femoral component in the sagittal plane is not possible because of using plane radiographs with a lateral view of the knee (the femoral head is not imaged). It also is variable to define a reference line5,23 tangential to a curve (curve = anterior bow of the femur). Two studies8,31 judged the optimal flexion/extension position of the femoral component perpendicular to the femoral mechanical axis. Implanting the femoral component strictly parallel to the mechanical axis can lead to an anterior cortical notching, especially in a femur with a large anterior bow (Fig 5B).
We found no difference in surgery time between groups. When using the imageless navigation module, no time consuming CT scan and extensive preoperative planning procedure had to be performed. Surgery time can be reduced to a mean of 70 to 80 minutes after an improvement in the special navigation cutting guides and optimization of the software work flow.
We found no differences in radiographic alignment of the implanted total knee prostheses using the CT-based or imageless navigation module. A high precision of implantation can be achieved by using CT-assisted surgical techniques for TKAs. At 2 years followup, we found better ligament balancing results in patients in the imageless study group. All other clinical variables showed no differences. Both methods offer comparable radiographic results and clinical outcomes. We prefer the imageless module because it is less time consuming and less expensive, whereas patients prefer the imageless module because they receive less radiation.
We thank C. Sukopp, MD, Munich, Germany, for radiographic and clinical measurements and J. Hayden, MSN, RN, Department of Orthopedic Surgery, Rush University Medical Center, Chicago, IL, for statistical analysis.
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