Unicompartmental Knee Arthroplasty: Indications and Technique
Anteroposterior, lateral, and patellar view of a 66-year-old patient with left knee pain that is recalcitrant to nonoperative measures. There is lateral joint space narrowing, preserved medial compartment, and minimal disease in the patellofemoral compartment
After a discussion about the risks and benefits of surgery, shared decision making with the patient led us to recommend a lateral unicompartmental arthroplasty. We reached this conclusion with this particular patient because she prioritized her active lifestyle and a quick recovery in our discussions. The patient was then scheduled for preoperative optimization, teaching, and finally outpatient surgery.
In preparation for a lateral unicompartmental arthroplasty, the surgeon must plan the approach to the knee. A lateral arthrotomy allows for a smaller incision and less soft tissue disruption, but the surgeon should be comfortable performing a total knee arthroplasty through this approach should it be required. An alternative is to perform an arthroscopy prior to incision to determine whether lateral unicompartmental arthroplasty is an option. Lastly, if the surgeon is not comfortable with a lateral arthrotomy , a medial arthrotomy with a larger dissection may be chosen.
The lateral side of the knee has increased translation and overall laxity than the medial side. For this reason, it is universally accepted that a fixed bearing implant be used. Mobile bearing implants have a propensity to dissociate when used for lateral UKA . Secondly, it is important to identify whether the chosen system has implants specific to the lateral side of the knee or if the system has “left lateral, right medial” implants. In addition, the surgeon should be sure that the bone loss on the lateral side can be managed with the available polyethylene thicknesses for the chosen system. Finally, should there be undiagnosed osteoarthritis within the medial or patellofemoral joints, or an incompetent ACL, a TKA system should be readily available as should the necessary retractors to perform the case for the chosen approach.
Adequate exposure can be achieved from either a medial or lateral approach to the knee. The benefits of a medial approach include familiarity, ease of creating the vertical tibial cut adequately medial, and ease of conversion to a total knee arthroplasty. Lateral UKA from the lateral side minimizes incision length, soft tissue dissection, and allows for greater visualization.
The authors prefer the lateral approach for a lateral UKA. The skin incision begins at the superior-lateral pole of the patella and extends to the lateral edge of the tibial tubercle approximately 1 cm distal to the joint line (Fig. 9.2). This incision can be extended proximally should greater exposure be required. The skin incision is made with care as the subcutaneous tissue on the lateral side of the knee is thin between the skin and the lateral retinaculum.
The arthrotomy is made beginning 1 cm superior to the superolateral pole of the patella, unlike the medial side where the vastus medialis can be entered as part of the arthrotomy; on the lateral side, the arthrotomy is solely in the capsule as the vastus lateralis is not close the incision. The arthrotomy then extends distally parallel and just lateral to the patellar tendon. Care must be taken not to violate the patellar tendon with the arthrotomy as the patellar tendon may be more lateral than the surgeon expects if unfamiliar with a lateral arthrotomy. The arthrotomy is kept as medial as possible to achieve adequate exposure to perform the operation. The retinaculum and synovium are thinner on the lateral side of the knee, so care of these structures should be taken in order to have adequate tissue for arthrotomy closure. The fat pad is larger laterally than medially and will require some resection for exposure; resecting it just lateral to the lateral tibial spine is effective. After resection of the anterior horn of the lateral meniscus, the knee is placed in approximately 30–40 degrees of flexion and a retractor is placed in the intercondylar notch to inspect the patellofemoral joint and the medial compartment. Should the surgeon be uncomfortable performing a total knee arthroplasty from a lateral arthrotomy, an arthroscope may be used to inspect the medial side of the knee before the arthrotomy is performed. Finally, exposure to the lateral compartment is completed by releasing the coronary ligaments and lateral capsule from the lateral plateau in a subperiosteal fashion in order to place a retractor laterally to avoid damage to the lateral soft tissue structures (Fig. 9.3).
A medial arthrotomy to the knee is also a viable option for surgeons who prefer familiar anatomy and the ability to convert to a TKA. The trade-offs include larger incision and dissection, added difficulty of accessing the lateral aspect of the lateral compartment, and finally, the need to work around the patella and keep it protected throughout the case.
Osseous Preparation and Soft Tissue Balancing
The authors prefer first preparing the tibia in unicompartmental arthroplasty; this then allows the flexion and extension gaps to be balanced as part of the femoral preparation. An extramedullary tibial cutting is placed perpendicular to the mechanical axis of the tibia. Care should be taken, based on implant system, to match the tibial slope, adjusting the slope of the guide in conjunction with the built-in slope of the cutting slot. Angulation of the cutting guide should be set by placing the distal aspect of the guide 5–10 mm lateral to the center of the ankle, allowing the guide to be parallel to the long axis of the tibia. Depth of resection should be approximately 1–2 mm off of the deepest aspect of the lateral plateau. Careful measurement of the depth of resection is important on the lateral side of the knee because lateral degeneration often creates more bone loss; overresection of the tibia can result in needing a large polyethylene insert, which may not be offered in the unicompartmental system.
After guide-pin placement, resection and slope is confirmed through the slot with a checking shim (angelwing). At this time, a lateral retractor placement is confirmed so that the lateral soft tissue structures are protected. The lateral tibial plateau is then resected, taking care not to undermine the tibial footprint of the ACL. A sagittal cut is then performed to complete the resection. The cut is parallel to the femoral condyle and the lateral tibial spine (Fig. 9.4).
Unlike the medial side, where the sagittal cut abuts the fibers of the ACL, the lateral tibial plateau is wider medial to lateral than the medial plateau and the implant will overhang in an anterior-posterior direction before it does in the medial-lateral direction. Therefore, it is not necessary to get all the way over to the fibers of the ACL and PCL , at the apex of the lateral tibial spine. In addition, on the medial side, the medial femoral osteophyte helps place the sagittal saw more lateral increasing the medial-lateral distance, which is helpful on the medial side. While on the lateral side, it is not necessary to have the sagittal saw more medial to increase the already large medial-lateral dimension; therefore, it is helpful to take out the lateral femoral osteophyte prior to making this sagittal cut (Fig. 9.5).