UKA with patellofemoral arthroplasty provides an alternative to TKA in select patients
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Bicompartmental knee arthroplasty is one of the hottest topics in knee replacement surgery.
Treating end-stage medial or lateral tibiofemoral osteoarthritis and patellofemoral OA with a unicompartmental knee arthroplasty in combination with a patellofemoral arthroplasty (PFA) has several advantages over total knee arthroplasty including ligaments and soft tissue sparing, bone stock preservation, reduced blood loss, faster recovery, reduced hospital length of stay, reduced complications and better functional outcomes. In particular, preserving both cruciate ligaments is advantageous for preserving native knee kinematics and proprioception, stair climbing and patient satisfaction.
Moreover, the anatomy, dimensions and the orientation of the condyles and of the trochlea are not standardized, but are related to morphotype, gender and race, and the condyles and trochlea differ among patients. Only a bicompartmental knee arthroplasty (BKA) allows a surgeon to choose the size and orientation of each component independently so that it reproduces the native knee anatomy and kinematics with a “custom-made” replacement.
Given the excellent midterm results and survivorship shown in recent studies, BKA may be considered a viable alternative to TKA in selected patients.
Indications for BKA
Indications for combined UKA and PFA are symptomatic lateral or medial tibiofemoral OA (Kellgren Lawrence grade 2 or higher), symptomatic patellofemoral OA (Iwano grade 2 or higher) and coronal deformity of the knee due to unicompartmental tibiofemoral wear (Figure 1). Other essential features of BKA are intact contralateral tibiofemoral compartment, ligament competence, range of motion (ROM) greater than 90°, flexion deformity less than 10° and no inflammatory disease.
Source: Matteo Marullo, MD
Generally, two different pathways of bicompartmental OA exist. In the first one, OA initially concerns one tibiofemoral compartment and secondarily involves the PF joint. In the second one, an initial patellofemoral OA (frequently secondary to trochlea dysplasia) is associated with a coronal deformity in varus or valgus that evolves into femorotibial OA.
Combined UKA plus PFA technique
The surgical approach is the same as for UKA, but it is extended 2 cm proximally. A medial parapatellar skin incision and a mini mid-vastus approach are used for medial UKA; a lateral parapatellar skin incision and capsulotomy through the lateral intermuscular septum are used for lateral UKA. I suggest not using the torniquet to avoid introducing a confounding factor in patellar tracking evaluation, to better control bleeding and to avoid local lactic acidosis.
Surgery should start with UKA to correct any coronal deformity and re-balance the forces acting on the patellofemoral joint. UKA is performed with the same technique as is used with an isolated UKA procedure. The UKA prosthesis should be implanted with a tibia-first technique, aiming for an anatomical alignment in the coronal plane and a consequent slight under-correction of the coronal deformity. A trick to understand the correct placement of the femoral component is to mark in full extension the center and the anterior edge of the tibial component on the femoral side. This mark will correspond to the anterior limit of the femoral component (Figure 2). If the correct dimension of the femoral component falls between two sizes, the smaller one should be used to avoid any interference or contact with the trochlear component of the PFA.
Dedicated UKA components
The medial and lateral compartments of the knee have different shapes: The lateral tibial plateau is round, the medial tibial plateau is “drop shaped,” the lateral femoral condyle is straight and the medial femoral condyle is curved. Consequently, the lateral and medial UKA should have dedicated components. The lateral UKA should have a round tibia and a straight femoral component; the medial UKA should have anatomical tibial and femoral components (Figure 3).
Once the UKA trial implant is in place, patellofemoral replacement surgery can start.
Patellofemoral “inlay” designs are anatomic, respect patellar tracking and are bone-stock preserving. “Onlay” or “trochlear-cutting” designs cover broader indications and are easier to implant than inlay designs. In 78% of cases, patellofemoral OA is secondary to trochlear dysplasia, so both the anatomy and patellar tracking must be corrected. In these cases, only a trochlear-cutting design can restore the adequate height of the hypoplastic lateral facet of the trochlea. A trochlear-cutting design is indicated, even in the case of patella alta, because it extends more proximally than an inlay PFA; consequently, it can restore adequate patellar tracking in the first, fundamental 30° of knee flexion without the need for any further surgical steps, such as tibial tuberosity distalization. For all these reasons, the inlay design should be used only in selected patients with a non-dysplastic trochlea and no major deformity.
Anterior femoral cut
PFA starts from the anterior femoral cut, which should be performed perpendicular to the sagittal axis of the joint. If the trochlear sulcus is present, drawing Whiteside’s line is helpful to identify the sagittal axis (Figure 4).
In case of primary OA, the thickness of the trochlear implant should replace the amount of bone and cartilage removed plus the cartilaginous wear. In patients with trochlear dysplasia, the height of the lateral facet must be created by undercutting the lateral side of the trochlea. In a case of high-grade trochlear dysplasia with a hypoplastic lateral facet, the anterior cut should be performed in slight external rotation to avoid lateral overstuffing and the need of lateral release. In any case, internal rotation should be avoided. With this trick, lateral release is only needed in a small percentage of cases.
After the anterior cut is made, the area where the trochlear component will be placed is prepared with a dedicated milling guide. The distal aspect of the guide should be flush with the articular cartilage both medially and laterally and its mediolateral width should cover the entire trochlea (Figure 5). The implant should not overhang mediolaterally to prevent any soft tissue impingement.
Smooth transition zone: UKA to PFA
The bed for the trochlear component should be accurately prepared to avoid any step in the cartilage-prosthesis transition zone. The distance between the trochlear component of the PFA and the femoral component of the UKA should be at least 2 mm to ensure adequate patellar tracking, even in deep flexion, and to prevent any patellar clunk or impingement. (Figure 6).
The patella is everted 90° to reduce mechanical stress on the patellar and quadriceps tendons and it is resurfaced with the same technique as is used for TKA. The patella must always be resurfaced to ensure proper patellar tracking. Cementation starts with the UKA implant and then is performed with the PFA implant (Figure 7).
Patellar tracking must be checked several times during surgery: during trialing, after cementation and with the knee capsule closed. The patella should be centered in the trochlea during the whole ROM, without any tilting, clunking or subluxation (Figure 8).
After administration of local infiltration analgesia is complete, the capsule and the skin are sutured. No drainage is required.
Patients start progressive weight-bearing 4 to 6 hours after surgery. Passive and active ROM is started consequently. Patients are discharged from the hospital orthopedic department on postoperative day 2 after demonstrating the ability to ambulate with crutches and to flex the knee at least 90° (Figure 9).
- References:
- Paratte S, et al. Clin Orthop. 2010;doi:10.1007/s11999-009-1018-0.
- Parratte S, et al. Orthop Traumatol Surg Res. 2015;doi:10.1016/j.otsr.2015.03.019.
- Romagnoli S, et al. J Arthroplasty. 2018;doi:10.1016/j.arth.2017.10.019.
- Romagnoli S, et al. Bi-unicompartmental knee protheses. In: Scuderi G, Tria A, eds. Minimally Invasive Surgery in Orthopedics. New York: Springer; 2016;doi:10.1007/978-3-319-34109-5_57.
- For more information:
- Matteo Marullo, MD, a knee and hip orthopedic surgeon, can be reached at Department of Joint Replacement, IRCCS Istituto Ortopedico Galeazzi, via R. Galeazzi 4, 20161 Milan, Italy; email: matteomarullo@hotmail.it.
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