Primary TKA is a successful operation and careful pre-operative planning to achieve the best outcome is required. The goal of primary total knee arthroplasty is to reestablish the normal mechanical axis with a stable prosthesis that is well fixed. This is achieved by bone resection,proper implant sizing and soft tissue balance.
The femoral component should be aligned with 5 to 10 degrees valgus angulation in the coronal plane and 0 to 10 degrees of flexion in the sagittal plane. The tibia should be resected at 90 ± 2 degrees to the long axis of the tibia in the coronal plane. In the sagittal plane, the posterior slope is dictated by the prosthetic design, but it appears preferable to recreate the posterior slope of the natural tibia.
Of prime importance is establishing equal flexion and extenion gaps . Anteroposterior stability depends on balanced flexion and extension gaps. Flexion gap is controlled by posterior cut of femur, tibial cut and posterior cruciate ligament. Extension gap is controlled by distal cut of femur, tibial cut and posterior capsule.
Soft tissue balance is necessary to provide optimum function and wear of the prosthesis.Simplistically, ligaments on the concave side of the joint need releasing and those on the convex side need tightening. Balancing must be achieved in both the coronal and sagittal planes.
A medial soft tissue release is made for a tight medial strap for a knee joint with varus deformity. The sequential procedures for medial release include the medial collateral ligament,posterior oblique ligament,semimembranosus tendon and pes anserine tendons .
For the knee with more severe varus deformity, resection of the bone along the medial tibial plateau done with downsizing and relative lateralization of the tibial baseplate.
A lateral soft tissue release is made for a tight lateral strap for a knee joint with valgus deformity. The sequential procedures for lateral release are (1) multiple punctured incisions of the iliotibial band and arcuate ligament release; (2) lateral collateral ligament release at the femoral epicondyle if the patient has tight soft tissue on knee flexion and extension, or iliotibial band release when tight soft tissue occurres on extension alone; and (3) popliteus tendon release at the femoral epicondyle.
If the valgus deformity is severe and this makes applying the cutting jig to the bone difficult, the contracture strap is released first and then the bone is cut.
A tight posterior strap is typically encountered with flexion contracture. For such a situation, a subperiosteal posterior strap release is typically performed along the posterior femoral condyle or a complete posterior capsulotomy is performed at the level of the joint line to increase the extension gap. More of the distal femoral bone is cut to increase the extension gap and ensure equal flexion and extension gaps. The femoral resection limit is defined as the site of origin of the collateral ligaments on the femoral epicondyles.
Revision TKA is much more complex and technically more difficult than first-time TKA. For a successful revision arthroplasty , one should clarify the cause of failure , use adequate surgical exposure , restore limb alignment , achieve soft tissue balance , use correct implant alignment , restore the joint line and obtain a good range of motion . |
