Unicompartmental knee arthroplasty (UKA) is a logical procedure when one compartment of the knee is affected. The success of the procedure will be equally influenced by the surgical technique and the choice of a proper indication. The indications for UKA are now clearly identified with (1) osteoarthritis or osteonecrosis limited to one femorotibial compartment ranked as Ahlback 4,1 (2) a deformity that is fully correctible on stress radiographs,10 and (3) a functional anterior cruciate ligament (ACL).14
The most recent evolution in UKA is the possibility to perform the arthroplasty using a surgical technique through a minimal incision. The goal of these techniques is to enhance the postoperative recovery, reduce the hospital stay, and accelerate the return to normal activities with appropriate knee function. We define such a technique as the ability to implant the components without an incision in the quadriceps tendon or the vastus medialis or lateralis (depending on the compartment to be replaced) and without everting the patella. Minimizing the trauma of the extensor mechanism should allow an earlier start of walking and active muscle exercises. Repicci and Eberle18 proposed this mini-incision for the implantation of unicompartmental components as a resurfacing procedure using limited instrumentation. With the evolution in instrumentation over the last 5 years, it is possible to perform partial knee arthroplasty with cutting guides fixed only on the replaced compartment preserving the integrity of the unaffected tibiofemoral compartment. As a result of these evolutions, we now routinely use the mini-incision for performing unicompartmental knee arthroplasty.2,15,17
We describe our surgical technique, which includes the limited approach of the replaced femorotibial compartment combined with the use of specific instrumentation dedicated to the mini-incision procedure. We also analyze the reported short-term clinical and radiographic results of UKA performed through a mini-incision.
The procedure can be performed under general or epidural anesthesia on a routine operating table and with the knee flexed 90° for skin incision, the thigh tourniquet inflated, and the foot resting on the table. However, using this type of mini-incision, frequent extension-flexion manipulations are necessary during the procedure because some structures are preferentially visualized at either low or high degrees of flexion.4
The length of the skin incision varies from 8 to 10 cm depending on skin elasticity and patient mass. It is important to maintain proper visualization throughout the procedure and this in part depends on the variation in tissue elasticity. Sometimes exposure is obtained by shifting the position of the knee, but in other cases, it might be necessary to extend the size of the incision to ensure correct positioning of the components. The upper limit of the incision is the superior pole of the patella extending distally toward the medial or lateral side of the tibial tuberosity but ending 2 cm under the joint line previously located. The proximal part of the incision is more essential for the procedure and two-thirds of the incision should be located above the joint line. Once the synovial cavity is opened, the part of the fat pad in the way of the condyle is excised and a dedicated curved, thin Homan retractor is placed on the medial or lateral side of the incision, protecting the collateral ligament. It is important to realize the principles of ligament balancing existing in TKA cannot be applied to UKA because the collateral ligaments should not be released in UKA. The proper tension of the ligaments will be restored in UKA while filling the gap left by the worn cartilage with the unicompartmental components.
The first step is to bring the knee to 60° of flexion to evaluate the joint by checking the resistance of the ACL with an appropriate hook and evaluating the state of both the opposite tibiofemoral joint and the patellofemoral joint (Fig 1). The osteophytes are then removed on the medial or lateral side of the femoral condyle, in the intercondylar notch to avoid late impingement with the ACL, and finally around the patella and the tibial plateau.
The next step is to progress to the bone cuts. In our practice, the tibial cut is always performed first both for uni- and TKA, but because the cuts are independent, the surgery can start with the femoral side as well. The operation can be performed either using extramedullary or intramedullary instrumentation. The extramedullary technique is based on the correction of the deformity of the leg in extension using an extramedullary rod that references the ankle and the femoral head. This will fix the direction and the level of the femoral distal cut performed first and link that parallel to the tibial cut made with the knee brought into flexion.
The tibial cut is made using an extramedullary instrument. The guide is placed distally around the ankle with the axis of the guide lying slightly medial to the center of the ankle joint. The proximal part of the guide is resting on the anterior tibia pointing toward the axis of the tibial spines and with modern instrumentation, it is possible to have the cutting part of the guide resting only on the upper tibia (medial or lateral) to be resected (Fig 2). The diaphyseal part of the guide is parallel to the anterior tibial crest, and the anteroposterior position of the guide is adjusted distally to reproduce the natural upper tibial slope, usually between 5° and 7° of posterior slope. The amount of resection is decided after using a probe located on the lowest part of the affected plateau. The horizontal tibial resection should reproduce the height of the nonaffected plateau. The sagittal tibial cut can be performed using one of the sagittal marks provided by the guide or made as a freehand cut aligned close to the tibial spine eminence, the anterior starting point decided after checking the alignment of the edge of the femoral condyle on the tibial plateau when the knee is brought close to full extension.
The entrance hole of the distal femur for the intramedullary technique is centered above the roof of the intercondylar notch. The drilling of the femoral medullary canal through a short incision often requires bringing the knee to lower degrees of flexion; otherwise, in flexion, the patella might induce incorrect alignment of the intramedullary guide. Once the guide has been properly introduced, the distal femoral cut can be made by reporting the angle between the anatomic and mechanical axis previously calculated on the full weightbearing view. This angle is usually 4° to 6°. It is critical to carefully protect the skin at the proximal part of the incision while performing this cut to avoid any skin damage. The amount of bone resected from the distal femur corresponds millimeter for millimeter to the femoral prosthesis. For valgus knees in case of femoral dysplasia, it is often necessary to use a more proximal distal femoral cut.
The remainder of the femoral cuts will then be completed using the appropriate cutting block. The positioning of this femoral finishing guide is the critical step of the procedure searching for the best compromise between an anatomically centered position on the femoral condyle and a long axis perpendicular to the resected tibial plateau. If an intramedullary technique has been used for the distal femoral cut, it is useful to have the patella retracted on the side by a retractor placed in the medullary canal (Fig 3). The retractor is introduced in the knee close to full extension and the joint is then brought to 90° of flexion. At that step, positioning of the anteroposterior cutting guide is critical to avoid any edge loading. Ideally, the femoral block should be slightly smaller anteriorly than the original femoral condyle. The use of tibial referencing based on the previously made tibial cut is probably the best landmark. Because the divergence of the medial condyle is different from one knee to another, checking the mediolateral position of the guide on the femoral condyle is also recommended. Using a minimal incision, the femoral cuts are usually performed more from inside to the middle of the joint rather than from anterior to posterior as commonly made with an open incision. Once the posterior cut has been made and the cutting guide removed, removal of any posterior osteophytes is necessary using a curved osteotome to increase the range of flexion and avoid any posterior impingement with the polyethylene in high flexion.
The size of the tibial tray should now be determined managing the best compromise between maximal tibial coverage and overhang, which might induce pain in the soft tissues. The anteroposterior size of the tibial plateau sometimes differs from the mediolateral one, especially for female knees, thus different sizing trials are necessary to find the best compromise. It is important to keep the depth of the tibial cut as conservative as possible to take advantage of the strength of the tibial cortex and the increased area of contact proximally. The knee is then brought into maximal flexion and externally rotated. The final preparation of the tibia is completed with the appropriate guide with the underlying keel impacted in the subchondral bone. Using a minimal incision, it is important to locate carefully the posterior margin of the tibial plateau to position correctly the keel in the anteroposterior direction. In case of hard cancellous bone, it might be useful to precut the future location of the keel using a reciprocating saw blade or an osteotome.
The flexion-extension gaps should be tested with the trial components in place and inserting a trial polyethylene liner. Common causes of impingement are residual bone eminence, incorrect position of the tibial or femoral component, or an oblique tibial cut. Once this has been verified, it is important at that step to look for a 2-mm protective laxity checked close to full extension to avoid any overcorrection of the deformity leading to progression of osteoarthritis in the unreplaced compartment. However, important residual varus deformity should be also avoided, as recently reported,19 to minimize the risk of polyethylene wear when using flat polyethylene inserts. The ideal correction as measured on the postoperative full weightbearing view will probably consist of a tibiofemoral axis crossing the knee between the tibial spines and the lateral third of the tibial plateau for a medial UKA.13
We cement all components for better fixation because long-term results suggest loosening is not a common mode of failure with modern cemented, metal-backed components.3,6 The tibial component will be cemented first with the knee in full flexion and externally rotated for a medial UKA. When cementing the components, it is important to avoid leaving any cement at the posterior aspect of the knee, and the use of a metal-backed tibial implant is useful to remove any posterior cement when using a minimal incision. Once the femoral implant has been cemented, bringing the knee close to extension helps remove any posterior cement with the polyethylene inserted last. Although all polyethylene tibiae may have some advantages for increased polyethylene thickness, disadvantages are the loss of posterior exposure during cementing and the impossibility of liner exchange if the polyethylene is worn through.
Patellar tracking should be checked before closing; the absence of patellar eversion during the procedure is helpful for that step (Fig 4). The tourniquet is released before closure for adequate hemostasis; in our practice, one intraarticular drain is left for 36 hours.
The goal of this surgical technique was to combine the advantages of the limited incision in the extensor mechanism18 with the precision of cutting guides routinely used to perform UKA.3,6 The novelty of this surgical technique, used now as a routine procedure, was the modification of the previously existing instrumentations to accommodate the requirements of limited access to the replaced compartment and the necessary precision of the guides allowing reproducibility of the technique.
Few reports are available at midterm followup for the evaluation of UKA performed through a mini-incision. After the initial description by Repicci and Eberle,18 Romanowski and Repicci evaluated, at 8 years followup, a group of 136 knees implanted with UKA performed through a mini-incision.20 The revision rate was 7%, including three of 10 for technical errors, and the revision time for osteoarthritis progression averaged 5 years.20 They suggested the procedure was only temporary to relieve pain and improve function with minimal morbidity. This suggestion contrasts with the long-term results reported for UKA performed through a conventional incision with a survival rate greater than 94% at 10 or 13 years.3,6 We believe the results and technical errors reported by Romanowski and Eberle20 highlight the difficulty performing the procedure through a minimal incision if the visualization is not appropriate. Hamilton et al11 recently evaluated a group of 221 UKAs performed through a mini-incision and compared these cases with a previous group of 514 knees operated on with an open technique. They reported an overall revision rate for aseptic loosening of 3.7% in the mini-incision group compared with 1% in the group with the open technique.11
There is probably less tolerance in UKA compared with TKA in component malpositioning, and it is therefore critical to carefully evaluate the radiographic position of components implanted with a limited incision to provide the same reproducibility as the open technique.8 In a randomized study presented by Carlsson et al,7 41 cemented Miller-Galante UKA knees were inserted through either an open or a minimal incision technique. The authors reported no difference regarding clinical or radiographic data with a mean postoperative femorotibial axis of 182° in both groups.7 They also performed a radiostereometric study of the components reporting a maximal total point motion of 0.8 mm for both groups at 2 years followup.7 When following the first group of 44 consecutive patients with 48 UKA knees implanted in our institution using a minimal incision, we found a progression of the mean Knee Society function score12 from 44 points to 94 points after 6 years followup. When studying the average femorotibial axis evaluated on long-leg films and the component position for frontal orientation or tibial slope, we found no difference compared with a group previously operated on with the open approach; however, one case of femoral divergence up to 9° was noted with the mini-incision technique.4 This case emphasizes the importance of correct visualization of the compartment to be replaced in UKA, and this is why the size of the incision might be different from one patient to another according to mass and soft tissue elasticity.
There is no doubt limiting or eliminating extensor mechanism disruption during the procedure provided more rapid recovery with less morbidity as mentioned in the original report by Price et al17 using the Oxford knee. They found the average rate of recovery was twice as fast compared with the standard incision.17 Pandit et al,16 evaluating a group of 101 Oxford UKA knees at 5 years, found 96% of the patients had good or excellent Knee Society scores. This accelerated recovery is associated with a decreased hospital stay in all the recent reports of UKA performed through a mini-incision.7,11,15,17 We have found patients can perform active exercises at 1 week compared with 3 weeks with the open technique. When full weightbearing was allowed in both techniques on the day after surgery, the walking activity was helped by crutches during 2 to 3 weeks with the open technique, whereas most of the patients were free to walk without any support at the end of the first week after UKA performed through a minimal incision.
In vivo kinematic evaluation of patients implanted with UKA demonstrates physiological knee function can be restored with femorotibial contact points and posterior femoral rollback during flexion close to the normal knee.5 The role of the ACL has also been previously highlighted for a well-functioning knee implanted with UKA.5,17 More recently, Fuchs et al9 evaluated the functional outcome of 29 patients implanted with a Repicci-type unicompartmental sledge prosthesis performed through a mini-incision. Using electromyography, gait analysis, and quality-of-life scores, they found UKA performed through a mini-incision led to functional parameters comparable to age-matched healthy individuals with a quality of life superior to TKA.9
All the clinical and functional outcomes reports tend to show quicker recovery with limited pitfalls for patients implanted with UKA performed through a mini-incision. However, no study has yet evaluated implant survival at 10 years, commonly considered as the acceptable threshold for joint reconstruction procedures.
Unicompartmental arthroplasty has become the standard of treatment for patients with full loss of cartilage limited to one femorotibial compartment. When the indications are satisfied and the surgical principles respected, it is possible to restore full knee function after the UKA procedure. The recent evolution in surgical technique and instrumentation has allowed the surgeon to perform the procedure through a reliable mini-incision approach. The midterm evaluation of UKA performed through a mini-incision suggests recovery is faster and morbidity is lower. Careful component positioning may require an extended incision according to patient specificity. Although long-term followup studies will be required to confirm acceptable clinical, radiographic, and survival results of UKA performed through a mini-incision, we believe there is no longer a place for extensor mechanism disruption or patellar eversion when performing a UKA.
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