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physical examination should include a full radiographic examination (eg, weight-bearing anteroposterior, lateral, posteroanterior 45 Rosenberg, and patellar views, and long leg alignment films). Radiographic inspection includes noting anteromedial joint line changes on lateral plain film radiograph for a varus knee (Ahlback stage 3 or 4) (Fig. 3) 28 and anterolateral joint line changes for a valgus knee (Fig. 4). Radiographic evidence of mild degenerative changes in the contralateral compartment can be present and still result in a successful UKA if the MRI does not reveal grade IV Outerbridge changes. Patellofemoral joint disease should be ignored regardless of Outerbridge classification if it is addressed with a patelloplasty intraoperatively.29,30 Tibial pseudosubluxation should be a red flag to avoid failure. Preoperative images allow visualization and measurement of angular deformities to determine the appropriate surgical plan and may even prove TKA to be the treatment of choice in some cases. Preoperative films are also used to template the prosthesis and to determine the mechanical axis for UKA to avoid overcorrecting alignment and risking failure. 88 Dunn et al A flexible, passively correctable, varus or valgus deformity of up to 15 on physical examination or stress view radiographs is acceptable, in our hands. In addition, a flexion contracture of up to 15 can be overcome in the operating room with an appropriate cut of the distal femur and tibia. A flexion range of motion (ROM) of 90can even yield an outcome of 130 or more in many patients, with return to sport in rare circumstances. SURGICAL TECHNIQUE Operative setup, skin preparation, and draping for UKA using an IM technique are similar to TKA. We require all patients to wash their whole body with Hibiclens or Phisohex 1 week before surgery. The Zimmer Unicompartmental High-Flex Knee System (ZUK; Zimmer, Warsaw, IN) has yielded excellent results in the Australian registry, outperforming its predecessor, the M/G (M/G; Zimmer, Warsaw, IN) at 1, 3, and 5 years in terms of revision rates.31 Many other accomplished surgeons have reported excellent midterm and some long-term results with this prosthesis. Fig. 1. (A) Osteonecrosis of the medial compartment. Preoperative posteroanterior notch view of avascular necrosis of the medial femoral condyle. (B) Medial compartment osteoarthritis. Preoperative anteroposterior view of medial compartment osteoarthritis. (C) Lateral compartment osteoarthritis. Preoperative posteroanterior view of lateral compartment osteoarthritis in a 58-year-old man. UKA: Intramedullary Technique 89 Fig. 2. The 1-finger test. On physical examination, the patient identifies the area of pain with 1 finger. Fig. 3. Lateral plain radiographs of a varus knee requiring medial unicompartmental knee arthroplasty. 90 Dunn et al SURGICAL APPROACH Minimally invasive surgical techniques are really a misnomer but have gained popularity with the general public. The size of implants restricts the ability to place the prosthesis with arthroscopic assistance. With the knee flexed to 45, the skin incision, 9 cm long, should be made approximately 1.5 to 2 cm (approximately 1 fingerbreadth) medial to the superior pole of the patella, extending to the tibial crest (Fig. 5), for a medial UKA. For a lateral UKA, the incision is biased laterally, 8 to 10 cm over the lateral compartment. The patient’s size should dictate the length of the incision to Fig. 4. Lateral plain radiographs of a valgus knee showing anterolateral joint line changes. Fig. 5. (A, B) Laterally based skin incision approximately 9 cm, being cautious of thin tissue. UKA: Intramedullary Technique 91 ensure accurate alignment of the prosthesis. The skin incision can be lengthened as needed for visualization if the superior aspect has a U shape, implying undue tension to the wound. The tension is relaxed at the superior or inferior apex of the incision to ensure good blood flow to the skin. Electrocautery should also be used as needed for hemostasis throughout the procedure. Hooded helmets with laminar flow can be helpful to ensure that the whole surgical team can attempt to limit any infections as well as protecting from blood being splashed inadvertently on the members of the surgical team. Dissection should occur in a single plane from the skin through subcutaneous fat and deep to Scarpa’s fascia before creating a medial tissue flap. This step preserves blood supply to surrounding soft tissues and decreases the risk of postoperative wound complications.32 Surgical approach does not seem to alter outcomes and should be dictated by surgeon preference.33,34 A quad-sparing parapatellar approach, a subvastus approach, or a midvastus approach can all be used for arthrotomy in a medial UKA, and a lateral parapatellar arthrotomy is used to enter the joint for a lateral UKA. We have found that by not violating the quadriceps tendon, our patients are out of bed the same day, with comfortable flexion in a continuous passive motion (CPM) device to 60. For a quad-sparing, midvastus approach, the arthrotomy should begin at the superior pole of the patella, dissecting in a single, full-thickness plane, and should extend proximally through the vastus medialis obliquus (VMO) or vastus lateralis obliquus (VLO) and repaired at the end of the case with an absorbable suture. We have not had
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