Introduction

Degenerative joint disease is a common diagnosis among adults, with an estimated prevalence of knee osteoarthritis ranging from 4.9% to 16.7%.^1,2 Though difficult to pinpoint exactly, there has been an increase in the prevalence of osteoarthritis among adults in the United States.¹ Patients often present with degeneration of multiple knee compartments, although the process can involve a single compartment. Laskin³ reported that less than 12% of patients in his practice had single-compartment disease and were candidates for unicompartmental knee arthroplasty (UKA). Moreover, isolated lateral compartment arthritis is quite uncommon and has received little attention in the orthopaedic literature. Scott⁴ reported that lateral UKAs constitute less than 1% of all knee arthroplasties. Despite its low prevalence, the aging and increasingly active population ensures that orthopaedic surgeons will manage patients with isolated lateral knee compartment arthritis. For some of these, lateral UKA is a predictable solution.

The cause of lateral compartment knee arthritis is likely multifactorial. Valgus knee deformity, genetic factors, traumatic events, and meniscal pathology may all play a role. Patients present with pain and often mechanical symptoms confined to the lateral aspect of the knee. The physical examination proceeds the same as for any patient with a degenerative knee process. Tenderness along the lateral distal femoral condyle and joint line are often noted. With advanced disease, the valgus deformity becomes more pronounced. Care should be taken to fully examine the knee to ensure the degenerative process does not involve the patellofemoral articulation or medial compartment. Likewise, ligament stability and the ability to passively correct a valgus deformity should be documented. Radiographic evaluation includes a weight-bearing anteroposterior film of both knees, a 45° flexed weight-bearing posteroanterior view, a lateral view, and a sunrise or comparable view of the patellofemoral articulation.

Indications

Orthopaedic surgeons entertain a host of management options for lateral knee compartment arthritis depending on patient characteristics and requirements. These options include nonsurgical modalities, osteotomies, total knee arthroplasty (TKA), or UKA. Our indications for lateral UKA are isolated lateral compartment degenerative changes and symptoms with intact knee ligaments and a passively correctable valgus deformity.

Contraindications

Contraindications include advanced degenerative changes of the medial or patellofemoral compartments, inflammatory arthritic conditions, anterior cruciate or other ligament insufficiency, fixed valgus deformity or deformity greater than 10°, less than 90° of knee flexion, flexion contracture of greater than 10°, or a patient incapable of adhering to a post-arthroplasty lifestyle.^5,6

Discussion

The potential benefits of UKA over TKA are faster recovery with shorter hospitalization and decreased morbidity, improved range of motion, better gait patterns, and bone conservation making future conversion to TKA easier should it be necessary.^7–10 Reports in the literature on medial unicompartmental arthroplasty have shown promising results.^11–14 Extrapolations of these data to the lateral compartment, however, may not be appropriate as it is quite different in terms of anatomic and biomechanical characteristics. There are limited studies that focus on the outcomes of lateral UKA.^5,10,15–18 Marmor¹⁵ was the first to discuss unicompartmental arthroplasty for lateral compartment disease. He reported excellent results in 11 of 14 cases. Ohdera et al.¹⁶ reported good results in 16 of 18 patients with greater than 5 years’ follow-up. Ashraf et al.¹⁷ reported on 88 patients with an average of 9 years’ follow-up and felt that the results of lateral UKA were comparable to those found with medial UKA. Pennington et al.⁵ reported on 29 knees among which there were markedly improved Hospital for Special Surgery knee scores and no revisions at greater than 12 years after lateral UKA. Early and midterm data suggest current-design lateral UKA offers reliable results.^5,10,16,17 As with any operative procedure, careful patient selection and adherence to proper indications and sound surgical technique influence these results. The remainder of this chapter reviews the preparation, surgical approach, technical factors, and postoperative management of lateral UKA.

Setup And Equipment

We utilize the same operating room setup as for a total knee replacement. The patient is positioned supine. A padded bump under the operative hip can be utilized at the surgeon’s discretion. A footrest or sandbag secured to the bed allows the knee to be flexed to 90° and beyond. Others drop the foot of the bed to use a suspended leg technique that allows hyperflexion of the knee.^19–21 A well-padded tourniquet is positioned high on the thigh, with the lower extremity prepared and draped in a manner to allow the surgeon control of the extremity. The procedure is performed under tourniquet for modern cement technique and to improve visualization. Equipment will vary depending on the chosen implant. It is important to have a total knee system available in the event that intraoperative inspection reveals advanced degenerative changes affecting the medial or patellofemoral compartments. Likewise, preoperative counseling should include a discussion of the possibility for total knee arthroplasty. Other considerations include specialized retractors. We like to use a retractor in the notch that extends over the trochlea to protect the patellofemoral compartment. The need for these instruments is dependent on the chosen approach.

 Surgical Approach (see Video 13-1)

The lateral compartment can be easily accessed through a slightly lateralized oblique, extensile, paralateral incision running along the lateral border of the tibial tubercle and the patellar tendon (Fig. 13–1). A modified lateral parapatellar arthrotomy is then performed. There is some concern that this adds complexity in the event that conversion to a TKA is required or that future revision surgery will be more of a challenge. Sah and Scott^6,18 reported this concern and described their technique for lateral UKA through a medial parapatellar approach. To provide another perspective, the lateral parapatellar approach has been quite successful in the senior author’s experience and has not posed problems intraoperatively or in the event of further surgery. The approach we describe provides excellent visualization of the surgical structures and minimizes soft tissue stripping.

Figure 13–1 Paralateral skin incision.

The skin incision is from the superior pole of the patella to just lateral to the tibial tubercle. The length of the incision is adjusted to allow for adequate visualization as needed. Skin flaps are full thickness but not excessively undermined. Once through the skin and subcutaneous fat, the arthrotomy proceeds through the retinaculum, staying lateral to the patellar tendon (Fig. 13–2). A moderate amount of fat is excised under the patellar tendon to allow for adequate visualization in determining tibial rotation. The lateral edge of the tibial plateau is exposed for visualization and to allow proper retractor placement by elevating a small portion of the iliotibial band off of Gerdy’s tubercle. The knee can be flexed and extended to allow visualization of the other compartments through this mobile soft tissue window. Final confirmation of isolated lateral disease is made prior to proceeding. Lateral femoral and tibial osteophytes should be removed. A patellar retractor is carefully placed after inspection of the anterior cruciate ligament to confirm its competence. A lateral Z retractor protects the iliotibial band and the lateral ligamentous and capsular structures after the visible portion of the lateral meniscus is excised.

Figure 13–2 Lateral parapatellar arthrotomy offering excellent exposure of the lateral compartment.

Technical Description

The exact steps of the procedure will vary depending on the chosen implant system, yet the principles are the same. We avoid mobile-bearing designs in the lateral compartment due to the risk of polyethylene dislocation.²² Ultimately, the goal is a measured resection of the distal femur and proximal tibia to allow for implantation of components with correction of the valgus deformity. Care must be taken not to overcorrect the knee and place undue stress on the medial compartment and lateral ligamentous structures as this may lead to accelerated wear of the medial compartment. Beginning with a “tibia-first” approach, an extramedullary alignment rod is utilized to perform a tibial resection adequate enough to accept the tibial component bearing surface. The flexion gap is usually a bit larger than that for a medial UKA due to the greater mobility of the lateral compartment. Coronal alignment should match the mechanical axis of the tibia and sagittal alignment should reproduce the natural posterior slope for each knee. A sagittal saw blade is utilized to make the initial cut vertical to the cutting block. This cut is in line with the medial border of the lateral femoral condyle and slightly internally rotated to account for the rotational relationship between the femur and tibia in full extension as a result of the “screw-home” mechanism. Care must be taken to avoid injury to the tibial insertion of the anterior cruciate ligament. The sagittal saw blade can be left in place to prevent undermining the anterior cruciate ligament tibial insertion while a standard saw blade is used to complete the tibial resection using the cutting block (Fig. 13–3). The resected tibial surface can assist in determining the appropriate tibial implant size. It is frequently shorter and wider than the resected tibia in a medial UKA due to the configuration of the lateral tibial plateau.

Figure 13–3 Extramedullary alignment rod and cutting jig for the tibial resection.

Distal femoral resection is accomplished by implant-specific techniques but is performed to result in equal flexion and extension gaps. It is important to note that balancing the flexion and extension gaps is accomplished on the femoral side, most notably on the distal femoral cut as the posterior cut is usually a measured resection specific to the implant. After measuring the flexion gap, the extension gap is cut in a manner to allow for the same-size spacer block to fit as comfortably as that used in flexion. In comparison with a medial UKA, this feel is slightly looser. Our preferred system utilizes the cut tibial surface to assist in alignment with the knee in extension. After the distal femoral resection is complete, a 2-in-1 cutting block is utilized to perform the posterior condylar and posterior chamfer cuts. Posterior femoral osteophytes are then removed, and the remaining meniscal tissue is excised. A femoral trial implant is selected to ensure adequate surface coverage without extending too anteriorly, which risks inappropriate articulation of the component with the patella. The cut tibial surface is likewise measured and a trial placed. The knee should move naturally through a range of motion without increased laxity or limitations to flexion and extension (Fig. 13–4). Stability is checked in full extension, 30° of flexion, midflexion, and finally full flexion (Fig. 13–5). Sagittal plane stability should also be confirmed. It is important to check motion and stability with retractors removed to allow for a true representation of ligament tension.

Figure 13–4 Trial components in knee extension (note proper component articulation).

Figure 13–5 Trial components in knee flexion (note proper component articulation).

Once satisfied with the construct, peg holes or keels can be prepared as per manufacturer recommendations, leaving the bone surfaces fully prepared for implantation (Fig. 13–6). The knee should be copiously irrigated with pulsatile lavage after areas of sclerotic subchondral bone are drilled with a small-diameter bit to improve cement interdigitation. A moistened surgical sponge placed with a tonsil forceps peripheral to the cut tibial surface aids in preventing retained lateral or posterior excess cement. With retractors positioned, the tibial implant is placed first. Cement technique is critical at this juncture as extruded posterior cement is difficult to retrieve with implants in place. It helps to place the posterior portion of the implant in first, then compressing it anteriorly so as to force the cement into the tibia and out anteriorly (Fig. 13–7). Extruded tibial cement is removed, as is the sponge. The femoral component is placed with a plastic tongue depressor sitting over the tibial baseplate to protect the femur from scratching (Fig. 13–8). We prefer an implant with femoral lugs that allow compression of the posterior condylar cement. Again, excess cement is removed and the polyethylene liner impacted into the tibial tray if using a modular design. The knee should be held in extension to slight flexion until the cement is hardened to prevent the tibial component from lifting up anteriorly. Final inspection of the implant should ensure normal knee motion and stability without patellar impingement (Figs. 13–9 and 13–10).

Figure 13–6 Prepared bone surfaces.

Figure 13–7 Implanting the tibial component from posterior to anterior, with surgical sponge placed to assist in excess cement removal.

Figure 13–8 Implanting the femoral component.

Figure 13–9 Final components in full extension.

Figure 13–10 Final components in full flexion.

Closure begins with deflation of the tourniquet and gaining hemostasis. The knee should be irrigated thoroughly as with any arthroplasty. The arthrotomy and skin are closed according to surgeon preference. A local injection of anesthetic and analgesic combination can assist with early mobility and postoperative pain management. A sterile dressing and lightly compressive wrap completes the case; surgical drains are not necessary.

Special Considerations

The unicompartmental arthroplasty of the lateral compartment is technically quite different than that of the medial compartment.²³ One of the more critical concepts unique to lateral UKA is the rotation of the tibial component. There is a tendency to introduce too much external rotation when preparing the tibia. The tibial resection is made with the knee flexed. As the knee extends, there is a relative external rotation of the tibia with respect to the femur. This screw-home mechanism results from the complex anatomy of the knee and the coupling of the cruciate ligaments.²⁴ Moglo and Shirazi-Adl²⁴ found the tibia to rotate internally 16.4° in 90° of knee flexion and externally 1.3° in knee hyperextension. If this tibial rotation is not taken into account during bone preparation, the tibial component will be externally rotated relative to the femoral component, causing edge loading of the polyethylene (Fig. 13–11). Pennington et al.⁵ identified this unique aspect of lateral unicompartmental replacements and offer recommendations on how to avoid it. Essentially, the key is in the rotation of the tibial implant, which must be accounted for when performing the tibial resection.⁵ By setting the tibial cutting guide in the appropriate alignment in flexion, the surgeon first ensures proper coronal plane alignment and reproduction of the posterior tibial slope. The vertical cut can be initiated after checking the orientation of this cut in full extension (Fig. 13–12). This will allow a better matching of the femoral condyle throughout the range of motion. If necessary, the femoral component can be lateralized slightly as well to improve articulation with the tibia.⁶ However, if the articulation is significantly off in extension such that there is edge loading of the implants, the likely problem is the tibial rotation. Though unproven in the literature, abnormal implant articulation likely would diminish the survival of the implant.

Figure 13–11 Initial tibial resection made in 90° of flexion and now visualized in full extension (note the improper rotation of the resected piece and the black ink mark indicating the proper rotational alignment of the tibial resection).

Figure 13–12 Sagittal saw resecting the tibia in the proper rotation.

Patients often remark how natural their knee feels after UKA. Preservation of the ligamentous structures of the knee, as well as the soft tissue, bone, and cartilage of the uninvolved compartments, likely provides proprioceptive feedback that account for this satisfaction. For the knee ligaments to function properly, it is critical to restore appropriate soft tissue tension without overcorrecting the deformity. If done correctly, ligamentous stability will be restored without overloading the uninvolved compartment. Trial components or spacer blocks are typically available with each system to assist in achieving the balance of a snug fit without requiring excessive force for insertion. Becoming familiar with the chosen implant system through training opportunities can reinforce the “feel” of properly balanced gaps for unicompartmental arthroplasty.

Postsurgical Management

Interdisciplinary management from pain management services, physical and occupational therapy, and case management allows for rapid recovery. Patients are allowed immediate weight bearing under the supervision of physical therapy. Peripheral nerve catheters managed by anesthesia specialists are quite helpful in promoting adequate pain control to accommodate a rapid recovery protocol. Such protocols facilitate early mobility and decrease length of hospital stay.²⁵ Patients are hospitalized overnight and typically released home on the first or second postoperative day when deemed appropriate by the multidisciplinary team (Figs. 13–13 and 13–14). Prophylaxis against venous thromboembolic events includes early mobilization and sequential compression devices. Most patients also receive aspirin 325 mg twice daily for 4–6 weeks. High-risk individuals are discharged on enoxaparin rather than aspirin at the discretion of the surgeon. Assistive devices are used for ambulation as required. Outpatient physical therapy concentrates on gait training, strengthening, proprioception, and range of motion. Initial follow-up in the office occurs 2–4 weeks after surgery. The ability for a patient to return to work is dependent upon job description and demands but can be quite soon for many office-type jobs.

Figure 13–13 Postoperative anteroposterior view of a 75-year-old female with isolated lateral compartment arthritis.

Figure 13–14 Postoperative lateral view of the same patient.

Summary

The lateral UKA is a rewarding operation for appropriate patients with isolated lateral compartment arthrosis, even though this is not a particularly common diagnosis. The procedure is well supported by early and midterm data on the topic; we expect long-term studies to better define the expectations from this procedure. Proper indications and surgical technique are bedrock principles for orthopaedic surgeons regardless of the procedure. When these principles are applied to current-design lateral unicompartmental knee arthroplasties, surgeons can expect satisfying results while delivering patients an excellent solution to their knee pathology.

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