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each remaining patient had a varus deformity, with an average pre-operative coronal planealignment of 4.6 ± 0.6º in the varus alignment < 10º group, 14.2 ± 0.3º in the 10º < varusalignment < 20º group, and 23.3 ± 1.0º in the varus alignment > 20º group.New TKA tensorAs previously described, our TKA tensor consists of three parts: an upper seesaw plate, a lowerplatform plate with a spike and an extra-articular main body (Fig. 1A)[14-18]. Both plates areplaced at the center of the knee. The PS TKA tensor consists of a seesaw plate with a proximalpost along the center that fits the inter-condylar space, as well as a cam for the femoral trialprosthesis. This post and cam mechanism controls the tibiofemoral position in both the coronaland sagittal planes. These mechanisms permit us to reproduce the joint constraint and alignmentafter implanting the prostheses.This device is ultimately designed to permit surgeons to measure the ligament balance and jointcenter/joint component gap, while applying a constant joint distraction force. Joint distractionforces ranging from 30lb (13.6 kg) to 80lb (36.3 kg) can be exerted between the seesaw andplatform plates through a specially made torque driver which can change the applied torquevalue. After sterilization, this torque driver is placed on a rack that contains a pinion mechanismalong the extra-articular main body, and the appropriate torque is applied to generate thedesignated distraction force; in preliminary in-vitro experiments, we obtained an error for jointdistraction within ± 3 %. Once appropriately distracted, attention is focused on two scales thatcorrespond to the tensor: the angle (°, positive value in varus imbalance) between the seesawand platform plates, and the distance (mm) between the center midpoints of upper surface ofthe seesaw plate and the proximal tibial cut. By measuring these angular deviations anddistances under a constant joint distraction force, we are able to measure the ligament balanceand joint center/joint component gaps, respectively.Intra-operative measurementWe performed all TKAs using measured resection technique with a conventional resectionblock. After inflating the air tourniquet with 280 mmHg at the outset of each procedure, weperformed a medial parapatellar arthrotomy. In all patients, the anterior cruciate ligament (ACL)and posterior cruciate ligament (PCL) were both resected. We performed distal femoralosteotomy perpendicular to the mechanical axis of the femur using preoperative long legradiographs. Femoral external rotation was preset at 3° or 5° relative to the posterior condylaraxis, which were determined by pre-operative computed tomography. After this, we performeda proximal tibial osteotomy, ensuring that each cut was made perpendicular to the mechanicalaxis in the coronal plane and with 7° of posterior inclination along the sagittal plane; there wereno bony defects noted along the eroded medial tibial plateau in any of these cases. Followingeach osteotomy, we removed osteophytes, released the posterior capsule along the femur, andcorrected any ligament imbalances that occurred in the coronal plane by releasing soft tissuesalong the medial structures of the knee according to the following criteria; (1) more than 20 mmof medial gap between the cutting surfaces of the femur and the tibia, (2) more than 10 mm ofjoint component gap, and (3) less than 5 cm of medial collateral ligament (MCL) release fromthe joint surface. In all knees with varus deformity, step by step appropriate release of medialside soft tissue (posteromedial capsule, MCL, semimenbranosus, and pes anserine tendons) wasperformed with a spacer block, in which residual lateral laxity especially at flexion was allowed.Following each bony resection and soft tissue release, we fixed the tensor to the proximal tibiaand fitted the femoral trial prosthesis. The joint distraction force was set at 40 lb. in all patients.We selected this distraction force because it re-creates a joint gap in full extension with femoraltrial which corresponds to the insert thickness of our preliminary clinical studies. We loadedthis joint distraction force several times until the joint component gap remained constant; thiswas done to reduce the error which can result from creep elongation of the surrounding softtissues. At this point, we measured the ligament balance (varus angle) (°) and joint componentgap (mm) with the knee at 0° (full extension), 10° (extension), 45° (mid-range flexion), 90°(flexion) and 135° (deep flexion), each with the patella reduced. For each measurement with areduced PF joint, we inserted a patellar trial prosthesis and temporarily repaired the medialparapatellar arthrotomy by applying stitches both proximally and distally to the connection armfile:///E|/ISTA2010-Abstracts.htm[12/7/2011 3:15:47 PM]