A 60-year-old female initially presented to the office complaining of worsening right knee pain dating back to a motor vehicle accident in 1975 that resulted in a shattered right a and subsequent patellectomy. Before injury, the patient had no right knee pain or extensor mechanism dysfunction. Postoperatively, the patient remained fairly pain-free with symptoms of quad/patellar tendon subluxation resulting in a non-painful pop over the lateral aspect of her distal femur during flexion including ascending and descending stairs that had been present since her patellectomy. Approximately 6 months before presentation, the patient noticed increased medial sided pain but denied any pain anteriorly or pain with subluxation of the patellar tendon. She was treated conservatively with physical therapy and a medial unloader knee brace but returned to the clinic 2 years later with worsening pain that began to significantly affect her activities of daily living. The patient has a body mass index of 20. Her past medical history is significant for diabetes and negative for tobacco use. The patient’s past surgical history is significant for patellofemoral arthroplasty on the contralateral knee. On examination, the patient ambulated with a normal gait and had 5/5 distal motor strength of the extensor hallucislongus, tibialis anterior, and gastrocnemius bilaterally. Sensation was intact to the L1 through S1 dermatomes. Knee range of motion was 0-135°, without extensor lag. The knee was stable to varus and valgus stress with 1A Lachman and posterior drawer tests. The patient had significant medial joint line tenderness to palpation, no pain laterally or anteriorly. On flexion, the patellar tendon subluxed laterally out of the trochlear groove at about 80°. Bilateral hip examination showed no abnormalities. Examination of the contralateral knee was unremarkable, with good clinical result status post-patellofemoral arthroplasty. The right knee radiographs, including anteroposterior, lateral, merchant, and posteroanterior views, revealed an absent patella, medial compartment narrowing with mild, correctible varus deformity of approximately 5° and medial joint line osteophytes (). Magnetic resonance imaging of the right knee confirmed that the anterior cruciate ligament (ACL), posterior cruciate ligament (PCL), medial collateral ligament (MCL), and lateral collateral ligament (LCL) were intact. There was diffusely exposed bone over the weight-bearing medial femoral condyle and tibial plateau. The trochlear sulcus was hypoplastic with exposed bone over the central portion and chronic remodeling of the anterior inferior margin. The lateral and patello femoral compartments were without any significant osteoarthritic changes with no chondral defects or degeneration. Finally, the patellar tendon was laterally subluxed over the anterolateral trochlear margin on extension views (). To track knee function preoperatively and postoperatively, the patient completed the previously validated International Knee Documentation Committee (IKDC) subjective knee evaluation form. After discussing the imaging results with the patient, we reviewed potential operative management following her failed course of conservative management. The risks and benefits of quadriceps realignment versus unicondylar knee arthroplasty versus total knee arthroplasty were discussed at length with the patient. We decided to proceed with a unicondylar knee arthroplasty of the medial compartment utilizing the MAKOplasty partial knee resurfacing system. The patient had compensated for her patellar tendon subluxation for >20 years, and the decision was made to leave the patellar tendon tracking alone rather than perform a soft tissue alignment which could have limited knee flexion, causing the tendon to rub, or abrade the trochlea and lead to a new source of pain and disability. At the time of surgery, the patient was placed in a supine position and a well-padded tourniquet was placed around the proximal right thigh. The limb was prepped and draped and a time-out procedure was completed along with confirmation of Ancef (Cefazolin) administration Two pins drilled in the tibia and femur for the attachment of the MAKO arrays. The limb was then exsanguinated and the tourniquet was inflated to 250mmHg. A median prepatellar approach was utilized and a 7cm incision was made along the anteromedial aspect of the knee. A minimal medial release was then completed. Retractors were placed and the appropriate data points were collected using the MAKO system array. The knee was put through a full range of motion and examined for stability as well joint space opening with valgus stress. The ACL, PCL, LCL, and MCL were found to be intact without instability. The lateral and patellofemoral compartments were examined and confirmed to be without significant degeneration. At this time, it was determined appropriate to proceed with the unicondylar knee arthroplasty instead of a total knee arthroplasty. Bone resection was completed on the femur and the tibia, and then, the trial components were placed with good were then stability. The patella tendon continued to subluxlaterally at approximately 80° of flexion, but no correction was performed as discussed with the patient preoperatively. The trial components were removed and the bony surfaces were copiously irrigated with pulse lavage. The appropriate components (size 3 femur, size 3 tibia, and 8mm polyethylene liner) were cemented (non-antibiotic loaded) into position beginning with the femur, then the tibia, and finally, insertion of the polyethylene after removal of excess cement. The knee was held in extension while the cement was allowed to harden. The final alignment was evaluated to be in neutral alignment with appropriate varus/valgus stability. No intraoperative pain cocktail or long-acting analgesic was utilized. A single ConstaVac drain was placed. The arthrotomy was closed, with the knee in approximately 30° of flexion, with figure-of-eight 0 Vicryl sutures. The skin was closed deeply with absorbable suture followed by running Monocryl stitch and Dermabond tape. A sterile compressive dressing was placed and the patient was taken to recovery after having tolerated the procedure without complication. Following the procedure, the patient was discharged with a home physical therapy program that focused on motion and muscle strengthening on postoperative day 1. Patient received 3 doses of IV antibiotics post- operatively, and the drain was removed prior to discharge. Postoperative pain control was managed with a 2-week course of narcotics. No post-operative antibiotics or deep vein thrombosis prophylaxis were prescribed. Upon her first postoperative visit at 2 weeks, the wounds were clean and dry without any evidence of wound dehiscence or infection. At her 5-week follow-up, the patient returned to the clinic with complete resolution of her pain. On examination, she had no medial joint line tenderness and range of motion from 0 to 120°. Radiographs of the right knee showed excellent alignment of medial components with restored medial joint space. Varus deformity was corrected to 1 degree of residual varus (). The patient returned at3months status post right knee medial unicondylar arthroplasty with significant improvement. On examination, she had no pain and symmetric knee range of motion, 0-140° bilaterally. She continued to have patellar tendon subluxation laterally at approximately 80° of flexion, stable from pre-operative evaluation. At 6months and 1year status post-surgery, the patient filled out an online version of the IKDC Subjective Knee Evaluation Form with stepwise improvement at each time point ().