Introduction: Computer navigation systems allow real time evaluation of knee behaviour intraoperatively. Measurements made by navigation reflect soft tissue balance throughout surgery. We studied three different populations of patients undergoing total knee replacement (TKR) with a CT-free navigation system where the goal was to achieve normal alignment. We compared the initial pathological kinematics in each group with the resultant kinematics after correction.

Method: The Orthopilot® was used during TKR for three groups of patients A (n=71), B(n=60) and C(n=43) all with endstage osteoarthritis. Patients in groups A and B had TKR performed by surgeon 1, and group C by surgeon 2.

Results: Pre-operatively, the mean mechanical femoral axis and the mean mechanical femoro-tibial (MFT) angle were calculated. The mean mechanical femoral axis for group A was −0.5° varus (−6° to 9°), group B was −0.68° varus(−6° to 6°), and for group C was 2.67° valgus (−12° to 10°). P< 0.0001, using Kruskal-Wallis test. Pre-operatively, the mean MFT angle for group A was −3.75° varus(−15° to 17°), group B was −2.98° varus(−17° to 13°), and for group C was 0.16° valgus(−17° to 25°). P=0.003 using Kruskal-Wallis test. These results show that the initial preoperative kinematics are different for the three different populations.

Post-operatively we measured the mean MFT angle in groups A, B and C. In group A, the mean MFT angle was −0.38° varus (−4° to 2°), group B was −0.41° varus(−5° to 2°), and group C was −0.02° varus(−3° to 5°). P=0.7 using the Kruskal-Wallis test. These results show that the post-operative kinematics are similar between the three different populations.

Discussion: Populations A and B preoperatively exhibited a mean varus MFT angle (−0.5° and −0.68° respectively), compared with a mean valgus MFT angle for group C(2.67°), which were statistically significantly different. Although different surgeons operated on the 3 groups (surgeon 1 operated on groups A and B, and surgeon 2 operated on group C), post-operative kinematics were within a narrow range (−0.02° to −0.41°) and not statistically different (p=0.7).

Conclusion: The Orthopilot® results showed that these populations had different initial pathological kinematics. Despite this, and using different operators we obtained similar post-op results within a narrow range. Computer navigation produces reliable, reproducible results independent of population or operator variables.

Introduction: Accurate soft tissue balancing is an essential part of total knee replacement (TKR), but has been difficult to quantify using traditional instrumentation methods. Computer navigation systems allow us to accurately assess intra-operative kinematics, which are affected by soft tissue management. The aims of this study were to evaluate the role of varus and valgus stress measurements and subsequently devise an algorithm for soft tissue management during TKR.

Methods: We used the Orthopilot® CT-free navigation system during TKR for patients with primary end-stage arthritis. This was a prospective study with 71 patients collecting intra-operative kinematic data. 57 knees were varus, 13 valgus, and 1 well aligned.

Pre- and post-operatively, the surgeon applied a varus and valgus stress at maximum extension, recording the mechanical femorotibial (MFT) angle. There were no patellar resurfacings. We compared the kinematics of each varus knee. Based upon the kinematics and the surgeon’s experience the following medial releases were performed as usual and divided into three categories:

No release (limited medial approach).

Results: Pre-operatively, the mean MFT angle was −9.6° (−3° to −22°) with varus stress and −0.8° (4° to −11°) with valgus stress. Post-operatively, the mean MFT angle was −3.7° (−1° to −7°) with varus stress, and 1.1° (4° to −3°) with valgus stress. Using regressional analysis, there was a strong linear correlation between both varus (r=0.742, p< 0.0001) and valgus (r=0.771, p< 0.0001) stresses and the MFT angle.

With the following medial releases, these kinematics were found:

No release – MFT angle not less than −12° with varus stress, greater than 2° with valgus stress, and/or if extension deficit was not greater than 5°.

The results show that post-operatively, the mean MFT angle is maintained within a narrow range (−1° to −7° with varus stress, 4° to −3° with valgus stress). 5/57(9%) patients had a mean MFT angle of 6.4°(0° to 7°) with valgus stress, and were considered to have been over-corrected. There were no extension deficits.

Conclusions: Navigation allows us to quantify soft tissue balancing based upon the initial kinematics with varus and valgus stress testing. From these measurements, an algorithm was developed, which showed that an appropriate release was made in 52/57 (91%) patients, but this may require some adjustment to reduce the number of outlying results.

Computer technology allows real time evaluation of knee behaviour throughout flexion. These measurements reflect tibial rotation about the femoral condyles, patellar tracking and soft tissue balance throughout surgery. An understanding of intraoperative kinematics allows accurate adjustment of TKR positioning. We studied computer navigation with the femoral component aligned to Whiteside’s line.

We used CT free navigation during TKR for 71 end-stage osteoarthritic patients. Patients demographics: 29 right–42 left; 44 female −27 male; age 70.4 years (+/− 8.4); mean BMI 30.8 (+/− 4.7; 23.2–48.6); Oxford score: 43 +/− 7.7 (28–58). Preoperatively, 57/71 knees were varus knees, 1 well-aligned and 13 valgus; 75% were cruciate retaining and 25% were posterior stabilised knees.

During surgery the frontal femorotibial or Hip-Knee-Ankle (HKA) angle was measured from maximum extension through 30°,60° and 90° of flexion. Measurements of the femoro tibial angles (HKA) in 0°, 30°, 60° and 90° of knee flexion before and after TKR were collected. No patella was replaced. We compared the kinematics of each knee. Femoral component rotation was 2.06° external rotation +/−1.32° (−1°; 5°) referenced from the dorsal condylar axis. Analysis divided the 71 patients into three groups:

When the femoral component was placed between 1° internal rotation and 0° of external rotation (7 patients) HKA tended to flex into valgus.