Unicompartmental Knee Arthroplasty
Rafael J. Sierra
Mark W. Pagnano
Key Concepts
Medial unicompartmental arthroplasty (UKA) is a good option for those patients with isolated advanced medial compartment degenerative arthritis (best if bone on bone) with acceptable patellofemoral cartilage damage.
Compared with total knee arthroplasty (TKA), a UKA is typically a smaller operation, has a quicker recovery, and can deliver a little better function because both cruciate ligaments are preserved.
In an experienced surgeon’s hands, the long-term survivorship of UKA with either mobile or fixed bearing designs rivals that of TKA.
Fixed versus mobile bearing design choice depends on surgeon preference and relative experience with the technique.
Medial mobile bearing design is contraindicated in patients with an incompetent anterior cruciate ligament (ACL), previous upper tibial osteotomy, and when difficulty is anticipated in balancing the medial side appropriately.
For the mobile bearing UKA, we recommend obtaining a stress view in valgus or a PA flexion view to determine integrity of the lateral compartment and to determine that the varus deformity is correctable. A fixed deformity indicates a structurally shortened medial collateral ligament (MCL) and likely an incompetent ACL and should be contraindication to a mobile bearing UKA.
Some surgeons including one of the authors (MWP) will use a weightbearing PA flexion view of the knee as a surrogate for the valgus stress x-ray in determining the integrity of the lateral compartment.
Indications
A UKA is indicated in all age groups when isolated advanced bone-on-bone arthritis is present in the medial compartment.
The ideal candidate is a patient with a varus deformity that is passively correctable with good range of motion and minimal patellofemoral arthritis (>100° of flexion < 15° flexion contracture, <15° varus deformity, not severe patellofemoral arthritis especially if involving the lateral facet).
The best results of UKA have been reported in patients with anteromedial osteoarthritis and an intact ACL.
Sterile Instruments and Implants
Routine knee retractors
Surgeon’s choice of unicompartmental knee arthroplasty system (with instrumentation)
Positioning
Patient can be positioned supine on the operating room table.
For mobile bearing UKA, the leg is placed in a leg holder and draped free hanging at approximately 100° of flexion and with slight hip flexion about 20°.
Surgical Approaches
UKA can be inserted through different approaches to the knee. A medial parapatellar approach most of the time is not necessary and less invasive approaches can be used. This includes a subvastus or a midvastus approach based on surgeon’s expertise and preference.
Preoperative Planning
Depending on the surgical technique used, AP and lateral radiographs are sufficient to plan for implant placement. The lateral x-ray for a mobile bearing UKA will help determine the size of the femoral component to be used, and the lateral tibial radiograph could give some insight as to the size of the tibial component as well. The final decision of the tibial component will be made intraoperatively.
Bone Implant and Soft Tissue Techniques
Mobile Bearing Unicompartmental Arthroplasty
Femur First Technique
Verify that that ACL is intact. Verify that the lateral compartment cartilage does not have any central defects. Always consent for TKA.
Mark the entry of an intramedullary femoral guide approximately 1 cm anteromedial and superior to the intercondylar notch. Use the rod pusher to position the rod intramedullary (Figure 48.1). The IM rod is placed as a guide for flexion of the femoral component. The femoral component should be flexed approximately 10° from sagittal plane of the femur.
Preparation of the femur. Remove the femoral osteophytes. Mark the center of the femoral guide with a marking pen in the center of the femoral condyle (Figure 48.2). Alternatively you can use the technique described by Shakespeare et al by measuring the distance from the lateral aspect of the
medial condyle as 13, 14, or 15 mm for a small, medium, or large components respectively. This will ensure that the meniscal bearing is seated 2 mm lateral from the lateral aspect of the wall of the tibial tray. The curved femoral guide then is decoupled from the femoral guide and used to position and drill the femoral component in place (Figure 48.3). The femoral guide is linked to the intramedullary guide. This will allow appropriate positioning in the flexion and extension plane (Figure 48.4). Be sure the guide is seated on bone and make the drill holes that coincide with the marked center of the condyle. Drill the anterior smaller hole, make sure that the guide is in the center of the condyle, and then drill the 6 mm drill hole thereafter (Figures 48.5 and 48.6). Place the guide of the posterior femoral resection (Figures 48.7 and 48.8). Complete your posterior resection. Protect the ACL and MCL.
Figure 48.1 ▪ The intramedullary guide has been placed in the appropriate position above the notch. It can be used to retract the patella.
Figure 48.2 ▪ The center of the medial femoral condyle has been marked.
Figure 48.3 ▪ The curved femoral guide then is decoupled from the femoral guide and used to position and drill the femoral component in place.
Figure 48.4 ▪ The femoral guide is then coupled to the intramedullary guide using the linking guide.
Figure 48.5 ▪ The 6 mm drill is used to prepare the larger hole for the femoral guide.
Figure 48.6 ▪ The femoral guide holes match the center of the medial femoral condyle.
Figure 48.7 ▪ The distal femoral resection guide is placed and the posterior femoral resection is verified.
Figure 48.8 ▪ The posterior femoral resection should match the posterior femoral condylar implant if the sizing is correct and the resection has been performed without allowing the saw to deflect off the bone.
Mill the distal femur. Begin milling the distal femur with a zero spigot. Remove the subsequent osteophytes and remaining bone and then place your femoral component as a trial (Figure 48.9). Subsequent millings are performed after tibial resection. Apply the spherical gauge around the femoral trial (Figure 48.10). Place the extramedullary tibial guide parallel to the long axis of the tibia. The extramedullary guide will cut the tibia with 7° of posterior slope if against the anterior aspect of the tibia. Use the G-clamp to clamp the tibial guide with the zero resection bloc and pin the tibial guide in place with one nonheaded pin (Figures 48.11 and 48.12). In order to avoid overresection of the tibia, change the zero guide for the +2 as your initial resection. Remove the G-clamp and check for resection level with the C-guide. Start with your vertical resection, stay medial to the ACL, and aim the blade toward the head of the femur but also use your trial femoral component as your guide
of internal and external rotation of the tibial cut. Subsequently make your horizontal cut, assuring protection to the MCL and the tibial spine (Figure 48.13). Remove the tibial bone. Verify that enough bone has been removed. If necessary recut with the 0 cutting guide. Place a tibial trial and assess the flexion extension gaps. The 3 mm feeler gauge should be easily inserted and removed without much extra force into the flexion space at 100° of flexion (Figure 48.14). Remove the 3 mm feeler gauge and bring the knee into extension and determine the extension gap. Establish the extension gap with the knee in approximately 20° of extension rather than full extension because of posterior capsular tightening. The gap is usually smaller and a thinner, metal feeler gauge should be used (Figure 48.15). If the 1 mm gauge is unable to be inserted in extension, then the gap is considered to be zero but could also be tighter than zero.
Figure 48.9 ▪ The femoral component is then placed after the initial milling with the 0 spigot.
Figure 48.10 ▪ The appropriate sized spoon is then placed around the femoral component trial.
Figure 48.11 ▪ The G-clamp is then used to couple the spoon to the tibial guide to aid in determining the depth of tibial resection.
Figure 48.12 ▪ The tibial guide is then pinned at the appropriate depth.
Figure 48.13 ▪ First the vertical and then the horizontal cuts for the tibial bone resection are made.
Figure 48.14 ▪ The flexion space should fit a 3 mm feeler gauge with relative looseness.Related posts:
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