Introduction: Computer-assisted minimal invasive total knee arthroplasty (MIS-TKA) provides the patient with the advantages of computer navigated total knee arthroplasty (CN-TKA) and early mobilization and progressive rehabilitation because of minor soft tissue injury. Since conventional landmarks are not visible for the surgeon throughout the operation, he must rely on the data displayed by the computer navigation system. This study was designed to assure that MIS-TKA reveals identical mechanical accuracy as conventional computer navigated TKA while reducing rehabilitation time.

Materials and methods: The Stryker knee navigation system was used intraoperatively. There were forty patients included in this study: 20 consecutive patients received computer navigated TKA (Scorpio) and 20 consecutive patients received MIS-TKA (Stryker-MIS Scorpio). Preoperatively and 6 months after the operation all patients received orthoradiograms and a CT-scan of the knee to determine mechanical and rotational alignment. Intraoperatively data shown by the navigation system was recorded. The HSS knee score was recorded preoperatively and 6 months postoperatively. Length of skin incision, length of hospital stay and postoperative range of motion were also determined.

Results: Analysis showed that the mechanical axis improved in both groups to less than 1° of varus. Rotational alignment of the femoral component showed reproducible values of less than 2° of rotation without significant differences between the two groups. Also, ligament laxity was almost identical preoperatively and postoperatively in 0°, 45° and 90° of knee flexion. Length of skin incision was significantly shorter in MIS-TKA. HSS knee score improved from a mean of 76 points (range 57–96) preoperatively to a mean of 92 points (range 64–100) 6 months postoperatively for both groups. Patients from the MIS-TKA group revealed a quicker recovery time than patients from the CN-TKA group. ROM improved sooner after the operation in the MIS-TKA group, however after 6 months both systems reached the same ROM. The length of hospital stay was significantly reduced by the MIS-TKA group.

Discussion: The Stryker knee navigation system aids the surgeon to precisely optimize mechanical and rotational alignment and to avoid malrotation and axial malalignment not only in CN-TKA but also in MIS-TKA. We have found no significant differences in preoperative and postoperative ligament laxity between the two groups. The advantages of minimal soft tissue damage in MIS-TKA can be achieved without loss of accuracy and with a great benefit for the patient.

Introduction: Computer navigation in total knee arthroplasty (TKA) may assist the surgeon with precise information about ligament tension and varus/valgus alignment throughout the complete range of motion, but there is only little information about how much ligament laxity is needed and how much laxity is too much. In the current study we measured the mechanical axis and opening of the joint at different time points, in different degrees of knee flexion and with varus and valgus stress during the procedure of computer navigated TKA.

Methods: Forty-nine consecutive patients underwent a MIS computer navigated TKA. With the Stryker Knee Navigation System varus/valgus alignment and distraction/compression was measured in 0°, 45° and 90° of knee flexion immediate after digitalization of the knee and after fascial closure. Values were noted in a neutral position and with maximal varus and maximal valgus stress applied. Patients with posterior stabilized implants were compared to those with cruciate retaining implants. Patients with preoperative varus malalignment or valgus malalignment were compared to patients with straight preoperative mechanical axes.

Results: At the beginning of the operative procedure the mean mechanical alignment was 1.9° varus at 0° knee flexion, 1.5° varus at 45° knee flexion and 1.5° varus at 90° knee flexion. Patients showed a mean mediolateral joint opening of 6.1° at 0° knee flexion, 5.9° at 45° knee flexion and 4.5° at 90° knee flexion. After implantation of the knee prosthesis and fascial closure mechanical alignment was 0.3° varus at 0° knee flexion, 0° varus at 45° knee flexion and 0.2° varus at 90° knee flexion. Mean joint laxity was 3.4° at 0° knee flexion, 3.1° at 45° knee flexion and 2.3° at 90° knee flexion. There was more lateral than medial joint opening postoperatively in 45° and 90° knee flexion regardless of the prosthesis type implanted. Preoperative varus and valgus malalignment could be reduced to values identical with those patients with straight preoperative mechanical axes.

Discussion: Mean varus/valgus laxity after implantation of a MIS computer navigated TKA was lower than prior to prosthesis implantation. Varus/valgus laxity of an approximate total range of 1.5°–2° can be achieved at all measured degrees of knee flexion and seems to be the ideal laxity for TKA. Computer navigation in TKA can consistently reduce preoperative varus/valgus malalignment to a level comparable to patients with preoperatively normal mechanical axes. More lateral joint opening is found before as well as after implantation of the prosthesis in 45° and 90° of knee flexion. The type of prosthesis implanted seems not to effect postoperative joint laxity.

Introduction: One of the four pillars of successful total knee arthroplasty (TKA) is restoration of the joint line. In conventional TKA the surgeon does not have a tool to control the accuracy of joint line restoration intraoperatively. The present study investigates if the preoperative joint line can be restored using an optical navigation system for TKA.

Materials and methods: Patients from two Orthopedic Centers (Istituto Orthopedico Rizzoli Bologna, 51 patients; Orthopädie Samedan, 42 patients) received computer assisted TKA (Stryker Scorpio) using the Stryker Knee navigation system. Using the software delivered with the navigation system depth of femoral and tibial medial and lateral osteotomies were recorded. After definite prosthesis implantation medial and lateral femoral condyle height as well as tibial length including polyethylene inlay were also recorded. Varus/valgus alignment was additionally recorded before and after prosthesis implantation.

Results: After femoral osteotomie varus/valgus alignment was 0 degrees (Stdv. 0.6 degrees). Lateral and medial osteotomies were performed with a depth of 8.2 and 8.8 mm respectively. Tibial osteotomie was performed with 0 degrees of varus/valgus (stdv. 0.7 degrees). Lateral and medial tibial osteotomies were performed with a depth of 7.5 and 4.8 mm respectively. After definite prosthesis implantation femoral and tibial varus/valgus alignment was 0.2 degrees of varus (stdv. 0.8 degrees) and 0.1 degrees of varus (stdv. 0.8 degrees) respectively. The femur was lengthened by 0.2 mm (stdy. 3 mm) medially and 1.1 mm (stdy. 3.1 mm) laterally. The tibia was shortened after component implantation by 1.5 mm (stdy. 3.4 mm) medially and 1.1 mm (stdv 3.1 mm) laterally.

Discussion: With the use of the Stryker Knee Navigation System, we can reconstruct the preoperative joint line with reproducible accuracy after a TKA. When prosthesis component and polyethylene inlay thickness are known, osteotomies may be performed and corrected intraoperatively to restore the joint line.