Anteromedial osteoarthritis (AMOA), the primary indication of medial UKA, is defined by a reduction of 75% of the tibiofemoral joint space, associated with condylar or tibial osteophytes or grade IV disease. Function of the knee is relatively normal with an intact anterior cruciate ligament (ACL), reducible varus deformity, both clinically and on valgus stress radiographs. The lateral compartment is nondegenerative with no pain on the lateral side of the knee, less than 25% loss of joint space on stress radiographs, and a normal aspect of the cartilage found intraoperatively.

Varus deformity >10° and >15° of flexion contracture are not considered as absolute contraindications, but they are classically associated with ACL deficiency, or not correctable on valgus stress. In those conditions, UKA cannot be recommended.

Avascular osteonecrosis (AVN) is a common cause of knee pain both in elderly and younger populations. It is the second most common indication of medial UKA when medical treatment fails, in primary disease or following previous surgeries (arthroscopic débridement with drilling).

Diagnosis is based on radiographs and confirmed with MRI allowing locoregional extension map of the necrosis. UKA is possible only in isolated disease localized to the subchondral region of one compartment without extension to epiphyseal or metaphyseal bone. However, MRI can be misleading as to the severity of disease when extensive edema is evident, while adequate bone support needs still to be present for successful UKA.

As reported by Marmor et al,¹ UKA has been used with success in the treatment of spontaneous or idiopathic medial femoral condyle osteonecrosis with good results. Our own results have confirmed this indication with favorable outcomes at long-term follow-up.²,³

High body mass index (BMI) was considered as a contraindication of UKA because fixed-bearing all-polyethylene implants have been shown to have poor survival in obese patients (Fig. 40-1).

Recent series on modern metal-backed designs, whether fixed or mobile bearing, have shown excellent survivorship in obese patients, equivalent to normal weight patients, associated with fewer reoperations, reduced deep infection risk, fewer perioperative complications, and better functional scores including range of motion after UKA comparable or even better than with TKA (Fig. 40-2).⁴ UKA wear is more related to activity, rather than BMI.

In conclusion, obesity and increasing BMI are no longer considered as contraindications to a metal-backed UKA, as recently described in a new perspective on UKA indications by members of the Knee Society.⁵

In patients with AMOA, age is no longer considered as a contraindication to UKA⁶ because revision UKAs are not correlated to activity or age.

Young patients have a greater level of activity, including professional activities and sport. They also have a greater expectation for the surgery results to restore knee function. UKA could appear to be an attractive alternative in this age category, as a conservative “first” arthroplasty allowing a better knee function, reducing the rehabilitation time, and preserving sporting activity. Young patients should be educated on the risk of earlier degradation in intensive high-impact activity and the potential for future revision.

The status of the patellofemoral joint continues to be a source of debate concerning indications of UKA.

In cases of AMOA associated with full-thickness cartilage loss of the lateral trochlea and/or lateral facet of the
patella or in cases of lateral patellar subluxation, implanting medial UKA is contraindicated. Other conditions of the patellofemoral joint such as spurring and intraoperative degenerative aspect of the medial facet or trochlea, anterior knee pain on physical examination are not considered as contraindications in the knee with AMOA.⁷,⁸ Konan et al⁹ found that UKA improved pain and function in all patients, with medial patellofemoral disease not affecting outcomes, contrary to central or lateral grade III patellofemoral disease having lower scores.

Chondrocalcinosis and radiographic signs of calcium within the cartilage or meniscus are not contraindications to UKA. However, clinical inflammatory disease with popliteal cyst and/or synovitis effusion demonstrating of an active pathology is a contraindication.¹⁰

Posttraumatic arthritis secondary to lateral tibial plateau fracture malunion causes pain and limited function for patients who are often relatively younger than the degenerative population, and UKA potentially allows correction of the intra-articular deformation due to the tibia’s fracture. Lustig et al¹¹ reported, in lateral UKA, improvement of knee pain, function, and an excellent survivorship at 5 to 10 years, with 80% good results at 15 years. Despite the limited number of indications and the technical challenge, lateral UKA could be an efficient option to treat lateral arthritis secondary to fractures with malunion, relieve knee pain, and restore knee function. Furthermore, implant survival rates seem to be comparable or close to those obtained for lateral UKA implanted for primary osteoarthritis.

Joint infection and inflammatory disease should be considered as contraindication to UKA.

In case of previous high tibial osteotomy (HTO) leading to an extra-articular deformity, the risk of overcorrection exists when the intra-articular varus deformity is corrected with UKA and may generate early failure of the lateral compartment.¹²

Thickness of cartilage <25% in lateral and patellofemoral compartments may demonstrate global osteoarthritis in the knee with an increased risk of early failure due to progression of the disease.

ACL deficiency is not an absolute contraindication. Some authors reported good results and mid-term survivorship for medial UKA performed in ACL-deficient knee or with a concomitant ACL reconstruction.¹³,¹⁴ The laxity is evaluated clinically as well as on lateral X-rays with anterior stress. An anterior translation, on stress views, greater than 10 mm or a posterior saucer-shaped indentation, reflecting ACL deficiency, can be an indication to perform an ACL reconstruction at the time of the UKA in young patients. In those special cases of knees associated to anterior laxity, UKA may still be considered if the osteoarthritis disease is localized to the medial compartment without a posterior wear pattern and the frontal deformity is fully reducible. The surgical technique in those cases could be modified with reduction of the tibial posterior slope during the tibial cut to reduce the anterior tibial laxity.

Osteoarthritis secondary to lower limb malalignment due to an extra-articular deformity is a contraindication to UKA. The extra-articular deformity can be secondary to a metaphyseal or diaphyseal fracture or due to a congenital deformity of the distal femur (essentially for valgus with hypoplasia of the lateral condyle) or the proximal tibia.

In cases where there are no or minimal intra-articular deformity, UKA might overcorrect the bone wear leading to early failure on the opposite compartment and on the implant due to excessive force. In the case of monocompartmental osteoarthritis associated to an extra-epiphyseal deformity, HTO is recommended to correct the lower limb malalignment.

To resume indications of UKA, the “ideal” patient should have osteoarthritis or avascular necrosis localized to a single compartment, with a reducible intra-articular deformity (<10° in varus, <15° in valgus, and no flexion contracture > 15°). Age and weight have no impact in cases of modern metal-backed implant. In cases of AMOA, if preoperative investigations find spurring and arthritis of the medial facet and/or trochlea, UKA can be implanted. For the special case of ACL deficiency in patients describing no A/P instability, UKA is still a good option if the osteoarthritis is localized to the medial compartment without any posterior wear pattern. UKA is reserved for patients with an advanced osteoarthritis (grade III or IV). Studies have demonstrated poorer outcomes and survival when medial UKA is used in patients with milder stages of osteoarthritis with partial-thickness cartilage loss.¹⁵

Theoretical advantages of mobile-bearing UKA, which is free to slide only in the anteroposterior axis, are based on an increased area of femorotibial contact during flexion producing a lower load stress transmission to the polyethylene, and then a more durable implant fixation over time. However, some disadvantages such as technical difficulty with regard to ligament balance and alignment, an increased risk of dislocation, and impingement have been described.

In the literature, several meta-analyses, systematic reviews, or randomized studies have compared the clinical outcomes of fixed- versus mobile-bearing UKA. They found no significant differences in clinical and functional outcomes between the two implants. They did not identify any significant difference in the relative risks for aseptic loosening, painful progression of osteoarthritis, periprosthetic fractures, and revision rates.¹⁶,¹⁷,¹⁸

The main reasons for revision in fixed-bearing UKA are progression of OA and wear, while mobile-bearing UKAs are often revised because of aseptic loosening, progression of OA, and dislocation.¹⁹ Time to failure could be shorter for mobile-bearing UKA perhaps due to a greater susceptibility of mobile-bearing outcomes to technical errors.²⁰

Concerning polyethylene backside wear due to the mobility of the bearing surfaces, Burton et al,²¹ using a displacement-controlled simulator, found a significantly greater volumetric wear rate for medial and lateral mobile-bearing UKA as compared to the fixed-bearing design.

In conclusion, fixed-bearing UKA has similar excellent outcomes and at least equivalent survivorship to the mobile-bearing implants without the risk of dislocation, allowing the surgeon to focus on gap-balancing while ensuring slight undercorrection.

With fixed-bearing UKA, a greater degree of undercorrection is tolerated giving the surgeon a wider margin of error. Even if mobile-bearing UKA may better replicate a native knee kinematics, both implant designs have similar functional outcomes.

Cemented implants are still the gold standard in UKA, but the proportion of cementless prosthesis is growing. The advantages of cementless implant include reduced surgical time, reduced incidence of tibial radiolucencies, avoiding cementation errors leading to impingement, or wear from third-body cement particles. Cementless coatings could be composed of hydroxyapatite alone (Unix UKA, Stryker, Mahwah, NJ, USA), porous titanium with hydroxyapatite (Oxford UKA), or tritanium (Tritanium UKA, Stryker Orthopaedics, Mahwah, NJ, USA).

Few studies in the literature have compared cementless versus cemented implants from the same manufacturers. Schlueter-Brust et al²² compared 152 cemented medial Uniglide UKAs versus 78 uncemented and 10 hybrid UKAs. The cementless group had a 97.4% 10-year survival rate versus 95.4% (and no significant difference) for the cemented group. Inversely, Panzram et al²³ found a similar 5-year implant survival of 94.1% for cemented group compared to 89.7% for uncemented (no statistical difference) in the incidence of radiolucent lines at 5 years. The New Zealand Joint Registry (Eighteen Year report January 1999 to December 2016. Wellington, New Zealand; 2017) reported a low revision rate of 0.8/100 component years for the uncemented Oxford UKA, but the lowest revision rate was for the cemented ZUK Zimmer UKA at 0.54/100 component years. Randomized control trials reported lower incidence of radiolucent lines²⁴,²⁵ and better Knee Society Functional Scores for cementless UKA group versus cemented UKA.²⁴

A biomechanical study showed that cementless UKA bears a higher risk of periprosthetic tibial plateau fractures as compared to cemented implant UKA (in cases of extended vertical saw cuts and reduced bone mineral density), but no difference was observed in clinical settings.²⁶

The theoretical advantages of all-polyethylene implants are greater bone preservation for insert thickness <9 to 10 mm (which is approximately total thickness of the thinnest metal-backed fixed-bearing UKA), and reduced backside wear and implant cost.²⁷ Disadvantages are a worse stresses and strains transmission to the underlying bone with 1.8 to 6 times greater microdamage compared to the metal-backed designs²⁸ which could lead to increased risk of aseptic loosening and pain. The clinical outcomes of all-polyethylene UKA are mixed, with several studies showing excellent mid- to long-term survivorship, while other studies found inferior outcomes compared to metal-backed designs.²⁹,³⁰

Preserved range of motion, meaning minimum 100° of flexion, and no lack of extension are required to perform UKA. The coronal and sagittal laxities have to be evaluated, especially in the posttraumatic valgus knee. During
the varus/valgus stress test, the coronal deformation should be fully correctible. Assessment of the ACL should be interpreted with caution, as the pivot shift test may be limited due to pain and swelling in an arthritic knee (Fig. 40-3).

Varus >10° and valgus >15° are usually considered as contraindications for UKA. In cases where the deformity is not fully correctable, soft-tissue releases are required and thus a TKA should be performed. UKA implants are only dedicated to fill the gap left by the worn cartilage and restore natural tension of the collateral ligament.

Knee pain should be localized in one compartment and potentially being showed precisely by the patient (“finger sign”).

The radiological analysis systematically includes anteroposterior (AP) and lateral views of the knee, full-length radiographs in bipodal and single-leg stance, varus and valgus stress radiographs, and a skyline view at 45° of knee flexion. Radiographs assess full-thickness cartilage loss in the involved compartment and intact cartilage thickness in the unaffected compartment, and confirm that the deformity is fully reducible. The lateral view of the joint confirms the absence of anterior tibial translation greater than 10 mm (referencing the posterior edge of the tibial plateau) and also shows that tibial erosion is limited to the anterior and mid-portions of the tibial plateau, confirming that the ACL is competent. A skyline view of patellofemoral joint should also be completed to ensure that there is no lateral joint space narrowing at 45° of flexion. The presence of periarticular osteophytes is not an absolute contraindication for a UKA.

It is important to understand why patients are undergoing UKA. If the main motivation is to return to high-level sporting activities, then a UKA is not the appropriate solution.³⁵ Resistant pain and limitations in the daily activities are the only reasons to justify surgery, particularly for young and active patients. Patients must be prepared for surgery, including both physical and psychological preparation. The physical preparation includes maintaining range of motion to limit the risk of a postoperative knee contracture and to prepare the patient for the postoperative rehabilitation program. In addition, it is essential to optimize the quadriceps’ and hamstrings’ strength at the time of surgery. Each step of the postoperative days and goals of the rehabilitation have to be presented to the patients preoperatively in an effort to manage their expectations and include them in a fast-track recovery program reducing the length of stay.

Performing a successful UKA requires a slight undercorrection of the limb alignment and to provide optimal soft-tissue balancing restoration (2 mm of medial laxity) in flexion and extension. The mechanical axis should pass between the middle of the resurfaced compartment and the tibial spines.

Restoring an anatomical alignment is the key to prevent implant loosening and polyethylene wear. The objective is to ensure congruency between the femoral component and the surface of the polyethylene in both flexion and extension without overhang that can lead to edge loading, especially between 20° and 60° of flexion when maximum forces occur during weight bearing.

This allows the knee to remain stable and improve the implant’s survival.

Excessive slope increases tension on the ACL and the risk of tibial loosening. Insufficient slope leads to limitation of flexion. The objective is to match the patient’s native natural slope, and the final target is a slope of 5° to 7°.

On the tibia, the largest size that can be accommodated without overhang is desired to place the component on the cortical rim.

This technique is adaptable to a mobile-bearing or a fixed-bearing medial UKA.

The skin incision starts medially to the superior pole of the patella and ends medial to the tibial tubercle, with two-thirds above the joint line and one-third below (Fig. 40-4). A medial arthrotomy is performed through a para-patellar approach.

Anterior horn of the medial meniscus and a portion of the fat pad are removed to facilitate intra-articular visualization. Caution has to be paid during deep medial dissection to not damage the deep fibers of the MCL. Inspection of the lateral compartment and patellofemoral joint and testing of the ACL can be performed (Fig. 40-5). Osteophytes of the medial femur and of the intercondylar notch are removed.

Objectives of the tibial component position are orthogonal to the tibial mechanical axis and 5° of tibial slope. The rotation position is aligned to the flexion axis of the native knee kinematics.

The horizontal bone cut is performed with an extramedullary guide and has to be the minimal amount needed to fit the tibial tray. The extramedullary guide is aligned to the anterior tibial crest with a neutral varus-valgus alignment (Fig. 40-6).

A thin curved retractor is helpful to protect the MCL from the saw blade during the tibial cut.

A risk during the tibial cut is to place too much tibial slope, especially for obese patients with thickness of soft tissues.

The sagittal bone cut is made medially to the apex of the medial spine, in the same plane as the flexion axis of the knee from 40° to 100° of flexion (Fig. 40-7).

The spacer block, set to the appropriate thickness, is easily inserted in flexion to confirm adequate tibial bone resection depth (Fig. 40-8). If it is tight, posterior medial osteophytes should be removed and a small amount of posterior femoral cartilage has to be removed, before concluding that bone resection is not enough.