Unicompartmental Knee Arthroplasty

Friedrich Boettner, MD

Tom Schmidt-Braekling, MD

Dr. Boettner or an immediate family member has received royalties from OrthoDevelopment; is a member of a speakers’ bureau or has made paid presentations on behalf of DJO Surgical; serves as a paid consultant to DePuy, A Johnson & Johnson Company, OrthoDevelopment, and Smith & Nephew; has received research or institutional support from Smith & Nephew; and serves as a board member, owner, officer, or committee member of the OrthoForum GmbH. Neither Dr. Schmidt-Braekling nor any immediate family member has received anything of value from or has stock or stock options held in a commercial company or institution related directly or indirectly to the subject of this article.

Introduction

Unicompartmental knee arthroplasty (UKA) is indicated if degenerative changes are limited to one compartment of the knee. The surgical procedure replaces only the affected compartment, differentiating a medial and lateral unicompartmental knee replacement and the patellofemoral knee replacement (Figure 44.1). The medial unicompartmental knee replacement is the most common of these. Currently, there are 45,000 UKAs and 600,000 primary total knee arthroplasties (TKAs) performed in the United States annually. While the number of TKAs is growing at a rate of 9.4% per year, the number of UKAs is increasing at a rate of 32.5%.

The benefits of a UKA include less blood loss, quicker recovery, shorter hospital stay, increased range of motion (ROM), higher postoperative activity levels, and earlier return to work. UKAs have decreased infection rates and lower perioperative complication rates.

The main disadvantage of UKA compared to TKA is lower long-term survival rates at 15 years compared to TKA. However, more recent studies have shown excellent long-term survival rates of UKA in selected patients.

With improved implant designs and more careful patient selection, the overall results of unicompartmental knee replacement have improved; currently, 10-year survival rates reported by experienced surgeons are better than 95% and quite similar to TKA.

Surgical Procedure

Because the UKA replaces only one compartment, the other compartments have to be relatively unaffected by osteoarthritis. It is well known from the literature that the progression of the arthritis in the adjacent compartment remains the main failure mechanism of UKA.

Indication Criteria for Medial and Lateral UKA

Unicompartmental arthritis (medial or lateral compartment)
Weight: <200 pounds
Activity: no heavy labor or high-impact sport (e.g., long-distance running)
Pain: Be able to pinpoint the pain to the correspondent joint line (the “one finger sign”)
ROM: preoperative flexion >90°
Flexion contracture <10°
Varus and valgus deformity <10°

Contraindications

Inflammatory diseases (e.g., rheumatoid arthritis, crystalline arthropathy)
Anterior cruciate ligament (ACL) deficiency
Patellofemoral arthritis (lateral patella facet)
Severe anterior knee pain

Surgical Technique

The surgical technique for different implants can vary based on the individual implant. However, all medial unicompartmental knee replacements have some similar features:

The incision is smaller and dissection into the quadriceps tendon is avoided.
Before implanting a medial unicompartmental knee replacement, the surgeon needs to be certain of the integrity of the ACL and the cartilage in the lateral and patellofemoral compartment using radiographs or MRI.

Figure 44.1 Radiograph of a unicompartmental knee replacement (A) and a total knee replacement (B).
The medial release around the tibia is limited, and surgical correction of the varus deformity by means of a medial soft-tissue release is not attempted.
The femoral component should be lined up in order to center the femoral component on the tibial component throughout the ROM from extension to flexion.
Tibial component rotation is crucial, especially for mobile-bearing unicompartmental knee replacements. Usually, the center of the femoral head is used to align the tibial component.
The thickness of the plastic insert should be selected to allow for some minimal laxity of the medial joint space. Overstuffing the medial compartment and pushing the knee into valgus alignment should be avoided to decrease the risk of progressive lateral compartment arthritis. A preoperative valgus stress view can help to judge the degree of medial laxity.
Remove posterior condyle osteophytes and the meniscus to minimize impingement in flexion.
At the end of the procedure, evaluate the patella tracking to avoid patella lateralization. A lateral release can be considered in cases of maltracking.

Perioperative Complications

In general, the risk of perioperative complications—including deep implant infection, deep venous thrombosis (DVT), and pulmonary emboli—is reduced with UKA compared to TKA. Complications after UKA include:

DVT and pulmonary emboli: Prophylaxis with coated aspirin is recommended in combination with regional anesthesia.
The risk for perioperative blood transfusion is minimal; thus, routine preoperative autologous blood donations is not recommended.
Deep implant infection: Routine 24-hour antibiotic prophylaxis is enforced.
Instability and mechanical malalignment predispose to early progression of the arthritis in the lateral or patellofemoral compartment.
Postoperative stiffness is a rare complication.
Perioperative fractures have been described in the past, but are less common with modern instruments.

Postoperative Rehabilitation

In most cases, patients undergoing unicompartmental knee replacement are considered candidates for accelerated rehabilitation and early discharge. Preoperative education classes and prehabilitation can prepare the patient for the early postoperative rehabilitation. As part of these protocols, postoperative exercises and walking with a cane should be reviewed prior to surgery. The patient needs to be familiar with the desired postoperative exercises if the procedure is performed on an outpatient basis or a discharge is planned for postoperative day 1. The patient should develop a clear understanding of postoperative goals and the timeline for rehabilitation prior to surgery. This includes realistic planning of return to work and driving. In general, patients should be able to return to low-demand work, such as office-type jobs, within 1 to 3 weeks and can return to driving a car once they are off all narcotic
pain medications and have pain-free ROM and command over their knee. This is usually the case after 2 to 4 weeks. Communicating realistic expectations is crucial and of special importance for less-invasive procedures such as UKA since patients might be misguided by information presented on the Internet and other media.

Preoperative Exercises

Preoperative conditioning and strengthening is important for patients that plan to benefit from an accelerated postoperative rehabilitation. These exercises include:

General conditioning on a stationary bike or elliptical machine for 30 minutes as tolerated three to four times a week
ROM exercises to improve preoperative ROM
Quadriceps strengthening exercises: Closed-chain exercises: wall sits and mini-squats (0°–45°), as tolerated

Days 1 to 7

Pain Management

Multimodal pain management is crucial in the first week. It combines regional anesthesia, antiemetic medications, oral and intravenous nonsteroidal anti-inflammatory drugs (NSAIDs) and narcotic pain medications. During the first 7 days, narcotic pain medications should be utilized every 4 to 8 hours as needed.

Activities of Daily Living (ADLs)