Unicompartmental Knee Replacement for Varus or Valgus Malalignment

Indications

Tibiofemoral unicompartmental knee arthroplasty (UKA) is a viable option for patients in whom severe joint damage and complete loss of joint space is present in one tibiofemoral compartment. Although early reports of this procedure were disappointing, improved product materials and design, robotic technology, and a better understanding of patient indications have improved the expected outcome. The advantages of UKA over total knee arthroplasty (TKA) include lower morbidity because of less blood loss, smaller incisions, and decreased soft tissue injury; preservation of bone stock and preservation of normal (or nearly normal) knee kinematics that result in a return to higher levels of function, fewer complications, shortened hospital stays, and an overall faster recovery. A study of surgical trends of UKA and TKA from U.S. Medicare beneficiaries aged 65 years or older found a sixfold increase in the number of patients who underwent UKA from 2000 to 2009, compared with a 1.7-fold increase in the number of patients who had TKA during this same time period. Bicompartmental knee replacements have been described ; however, this chapter focuses on tibiofemoral UKA. Patellofemoral knee arthroplasty is described in Chapter 37 .

The primary indication for UKA is symptomatic isolated painful tibiofemoral arthritis. The procedure is performed in both men and women, with multiple studies demonstrating equivalent results between genders. Medial tibiofemoral osteoarthritis (OA) secondary to anterior cruciate ligament (ACL) deficiency is an indication in patients willing to undergo concomitant or staged ACL reconstruction, if instability is problematic. A few investigators reported no difference in outcomes and survivorship between patients who had an intact ACL and patients who had a deficient ACL without instability symptoms. Therefore ACL reconstruction may not be required if giving-way does not occur with activity. Lateral UKA is indicated for primary OA, such as that which develops after a lateral meniscectomy, and posttraumatic OA secondary to a tibial plateau fracture.

Patient candidates typically have moderate to severe joint line pain and/or stiffness that limits daily activities. Pain and swelling may occur while resting, either during the day or at night. A lengthy course of nonoperative treatment, including nonsteroidal antiinflammatory drugs (NSAIDs), steroid injections, physical therapy, and weight control, fail to alleviate the pain. In our opinion, the patient should be less than 60 years of age, although consideration may be made for those older than 60 years if no contraindications exist and the other knee compartments are not arthritic or symptomatic.

Most patient candidates for UKA will have undergone prior failed surgical procedures such as chondroplasty, meniscectomy, microfracture, autologous chondrocyte implantation (ACI), or osteochondral autograft transfer. In our experience, approximately 60% of the patients who underwent UKA have undergone these types of prior failed procedures. It is important that the patient have realistic expectations regarding what this operation may and may not accomplish. At our center, most patients are able to walk without support within approximately 2 to 3 postoperative weeks, and the majority return to daily activities within 4 to 6 weeks. Light, low-impact activities such as walking, swimming, golfing, light hiking, and bicycling can often be performed by 3 months. However, running and high-impact athletic or occupational activities are not advised and, in fact, should be avoided; and it is essential that the patient understand and accept these limitations. Recent studies have reported that the majority of patients who were participating in sports before UKA were able to return after surgery and that most participated in low-impact activities such as swimming, bicycling, and general fitness training.

Contraindications

The principal contraindication to UKA is preexisting joint arthritis in another compartment of the knee (moderate to advanced articular cartilage deterioration and/or moderate to severe joint space narrowing on standing posteroanterior [PA] radiographs) owing to the evidence that this is one of the leading causes of failure of UKA. Anterior knee pain, without patellofemoral joint narrowing or arthritic damage, is not an absolute contraindication. However, patellofemoral symptoms with stairs, kneeling, and light activity will persist after UKA, and most patients with these problems select TKA. Mild patellofemoral joint deterioration (Outerbridge grade 1-2) also appears not to be deleterious to short-term outcomes of UKA.

Other contraindications include uncorrected excessive varus or valgus malalignment (>3 degrees), knee ligament instability, knee hyperextension of 10 degrees or more, inflammatory arthritis, and prior infection. Patients with complex regional pain syndrome, diabetes, knee arthrofibrosis (excessive extension or flexion contracture), chondrocalcinosis, rheumatoid arthritis, or neuromuscular disorders affecting limb control are not considered candidates.

Patients who are obese (body mass index [BMI] >32) are not considered candidates for UKA, although recent reports noted no significant differences in 5- and 10-year survival rates according to BMI subcategories. Osteopenia (bone mineral density T score between −1.0 and −2.5 standard deviations below normal) or osteoporosis (T score less than −2.5) are contraindications for this operation owing to the potential risk of subsidence of the implants. Other contraindications include failure to treat the patient with all possible conservative measures, noncompliance with rehabilitation, and unrealistic expectations for future activity levels. Although our experience with this operation has usually involved patients younger than 60 years, age alone is not a contraindication to the procedure, assuming no previously mentioned contraindications exist. A few reports have shown poorer results when medial UKA was done in patients with only partial loss of tibiofemoral joint space and articular cartilage. Ultimately, the final decision to perform UKA or TKA is made at surgery after all of the joint surfaces have been visualized. The patient should be informed and consent obtained to perform either procedure.

Implant Design

UKAs may be generally categorized according to the bone-cut preparation (resurfacing or inset) and bearing surface (mobile or fixed, Tables 30-1 and 30-2 ). Resurfacing implants require minimal bone resection, and inset implants require angular cutting similar to TKA. Bearing surfaces are usually all polyethylene or modular and include fixed bearing and mobile bearing. The initial UKA implants, such as the Marmor (Smith & Nephew) and St Georg Sled (Waldemar Link), were fixed bearing with all-polyethylene tibial inserts. Problems with these prostheses included subsidence in both the femoral condyle and tibial plateau, aseptic loosening, and wear caused by suboptimal design. Subsequently, implants were redesigned to distribute loads onto the cortical rim and had a minimum thickness of the tibial insert of more than 6 mm. Metal-backed components were introduced, requiring greater bone resection. The Miller-Galante (Zimmer) is a well-known metal-backed, fixed-bearing implant with a modular polyethylene insert.

Mobil-bearing implant designs were introduced in attempts to reduce the stress exerted on the tibial surface. These implants, such as the Oxford (Biomet), have a metal femoral component that articulates with a polyethylene meniscal component and a flat metal tibial surface. A porous-coated prosthesis was developed to induce bone growth and provide better fixation. For instance, the cementless Oxford Phase 3 has a layer of porous titanium with calcium hydroxyapatite under its components. Mobile-bearing implants are subject to dislocation of the mobile insert from the tibial base.

Robotic Technology

It is well known and documented that early UKA failures were frequently caused by inaccurate positioning of the components, which led to undercorrection or overcorrection of the final limb alignment. Excessive malalignment of the tibial component (>3 degrees) or posterior tibial slope (>7 degrees) may cause component loosening, fractures, and increased bone stresses. Robotic-assisted surgical navigation was introduced in the early 2000s to improve accuracy in UKA (postoperative limb alignment, component positioning, and soft tissue balancing) using less invasive techniques. There are two types of robotic surgery systems: haptic (or tactile) and autonomous. Haptic systems require active participation of the surgeon to complete the entire operation, while autonomous robotic systems complete the operation after the surgeon has performed the approach and set up the machine. Haptic systems constrain the motion of the cutting tool to only the preplanned volume or area of resection. Preoperative 3-dimensional models of the patient's knee (created with computed tomography [CT] scans) are merged with referenced bony surfaces at surgery to form a final model of the actual anatomy of the patient. The component placement and exact cutting zones are determined. As the surgeon resects these areas, the robotic arm provides auditory and tactile feedback, limiting the tip of the rotating burr to just the predefined cutting area. The MAKOplasty Partial Knee Resurfacing System that uses the Robotic Arm Interactive Orthopedic System (RIO) (Stryker) and the Acrobot system (The Acrobot Company) are examples of commercially available haptic systems.

Many studies have compared short-term outcomes between various navigation and conventional UKA techniques ( Table 30-3 ). A systematic review by Nair and associates identified 15 studies published from 2003 to 2011 in which five different navigation systems were used. Overall, the navigation methods improved component alignment and position and reduced radiographic outliers compared with conventional systems. However, there were no differences in clinical outcomes, which is not surprising considering that 14 studies had follow-up of 2 years or less, and just one study had a midterm follow-up of 7 years. A meta-analysis was conducted by Weber and colleagues of 10 studies (level II and III) involving 258 medial UKAs implanted with navigation and 295 medial UKAs implanted with conventional techniques. The study reported that there were more outliers in the conventional group compared with the navigation group for all variables assessed, including mechanics axis (30% and 11%, respectively), femoral anteroposterior (AP) alignment (17% and 5%, respectively), femoral lateral alignment (41% and 18%, respectively), tibial AP alignment (14% and 8%, respectively), and tibial slope (22% and 9%, respectively).

In the largest study to date, Mofidi and coworkers calculated accuracy rates by comparing intraoperative planned and postoperative radiographic measurements in 232 knees that underwent medial UKA using the MAKOplasty system. Accuracy rates of the femoral prosthesis were 2.8 ± 2.5 degrees in the coronal plane and 3.6 ± 3.3 degrees in the sagittal plane. Accuracy rates of the tibial component were 2.2 ± 1.75 degrees in the coronal plane and 2.4 ± 2 degrees in the sagittal plane. The authors concluded there was a high degree of agreement between intraoperatively planned prosthesis alignment and postoperatively measured alignment. Proper cementation technique was considered crucial in achieving accurate alignment.

Clinical Examination

A thorough history, including documentation of prior surgical procedures, conservative treatment measures, and all knee joint injuries, is required. Symptoms are typically experienced most often during stair climbing, walking on uneven ground, and kneeling or squatting.

The comprehensive physical examination includes a complete patellofemoral examination, discussed in detail in Chapter 35 , to assess patellar tilt, subluxation, mobility, Q angle, and lower limb rotational alignment (femoral internal torsion, tibial external torsion). The patella and all surrounding tissues are palpated to localize pain. The medial and lateral joint lines are also inspected for any tenderness, indicative of tibiofemoral joint involvement.

All knee ligament tests are conducted as described in Chapters 7 and 17 .

The patient's gait, range of knee motion, lower extremity and hip muscle strength, and neurovascular status are evaluated.

Radiographs taken during the initial examination include standing AP at 0 degrees, lateral at 30 degrees of knee flexion, weight-bearing PA at 45 degrees of knee flexion, and patellofemoral axial views. Double-stance full-standing radiographs of both lower extremities, from the femoral heads to the ankle joints, are obtained in knees in which varus or valgus lower extremity alignment is detected on clinical examination. Magnetic resonance imaging (MRI) is required in certain cases to determine the cartilage status of the entire knee joint, along with all other soft-tissue structures.