BACKGROUND: Unicompartmental knee arthroplasty of the medial compartment has excellent long-term clinical outcomes. Arthritis isolated to the lateral compartment is much less common; subsequently, the clinical outcomes of the treatment of that condition are less frequently reported. Most commonly, the lateral compartment is approached through a lateral arthrotomy. The purpose of this study was to determine the midterm results of lateral unicondylar replacement through a medial arthrotomy in patients with primary osteoarthritis or posttraumatic arthritis.
METHODS: From 1991 to 2004, forty-nine lateral unicompartmental knee arthroplasties were performed in forty-five patients by a single surgeon. One patient was excluded from the study because of a severe underlying neurologic condition. Lateral unicompartmental replacement was performed in thirty-eight knees with primary osteoarthritis and in ten knees with posttraumatic arthritis secondary to a tibial plateau fracture. Retrospective chart reviews and radiographic evaluations were performed, and Knee Society scores were determined.
RESULTS: The average Knee Society knee and function scores improved from 39 and 45 points, respectively, preoperatively to 89 and 80 points at an average of 5.2 years postoperatively. Preoperative alignment averaged 10° of valgus, which was corrected to an average of 6.2° of valgus postoperatively. There were no revisions and no notable soft-tissue complications. The mean postoperative knee and function scores were significantly better for patients with primary osteoarthritis (95 and 86 points, respectively) than they were for those with posttraumatic arthritis (74 and 65 points).
CONCLUSIONS: Lateral unicompartmental knee replacement through a medial approach provided durable and reliable short to midterm results. This approach is safe, effective, and extensile, making it a viable alternative to a lateral approach. The outcomes of lateral unicompartmental replacement in patients with posttraumatic arthritis can be expected to be inferior to those in patients with primary osteoarthritis.
LEVEL OF EVIDENCE: Therapeutic Level IV. See Instructions to Authors for a complete description of levels of evidence.
ORIGINAL ABSTRACT CITATION: “Lateral Unicompartmental Knee Arthroplasty Through a Medial Approach. Study with an Average Five-Year Follow-up” (2007;89:1948-54).
Arthritis isolated to the lateral compartment of the knee is less common than its medial counterpart, and it represents only 5% to 10% of all unicompartmental knee arthroplasties1,2. Consequently, clinical outcomes of lateral unicompartmental knee arthroplasties are less frequently reported and have more variable results3-7. Historically, the lateral compartment is most commonly approached through a lateral arthrotomy for this procedure. Despite its potential advantages, the lateral approach is much less frequently used for knee arthroplasty and it is technically difficult8,9. In contrast, a medial arthrotomy is a more familiar, safe, effective, and extensile approach, likely making it preferable for a wide range of surgeons. Our experience suggests that lateral unicompartmental knee replacement through a medial approach provides durable and reliable short-term to midterm results.
Preparation and Approach
The type of anesthesia is determined by patient preference, in consultation with the anesthesia staff.
The involved lower extremity is prepared and draped in the usual sterile fashion. A tourniquet is inflated after the limb is exsanguinated. Landmarks, including the patella, patellar tendon, and tibial tubercle, are outlined. The surgical technique consists of a standard median parapatellar arthrotomy performed through an incision approximately 10 cm in length placed slightly medial to the midline (Fig. 1).
A medial arthrotomy is a familiar approach that allows simple conversion to total knee replacement if unicompartmental knee arthroplasty is abandoned intraoperatively, which can occur in up to 50% of candidates for lateral unicompartmental knee arthroplasty (Fig. 2)10. This technique provides excellent visualization of the lateral compartment as well as the remaining compartments of the knee. An appreciation of knee anatomy, an opportunity for minor débridement, and both patellar subluxation and eversion are possible with this approach (Fig. 3). A medial arthrotomy, if reused for revision surgery at a later time, is also associated with fewer complications11. Moist towels are used to protect the subcutaneous tissue and surrounding soft tissues throughout the procedure (Fig. 4).
As the arthrotomy approaches the anterior horn of the medial meniscus, the dissection is taken laterally and anterior to the coronary ligament to avoid injury to the medial meniscus (Fig. 5). The patella is subluxated, or everted if necessary, and the knee is flexed (Fig. 6). Resection of the fat pad is performed only as necessary for improved visualization for the lateral tibial resection. The medial and patellofemoral compartments are thoroughly evaluated at this point to reaffirm the intention to perform unicompartmental knee arthroplasty, or to abandon it and proceed to total knee arthroplasty. Inspection and minor débridement of the medial and patellofemoral compartments are facilitated with this approach. Osteophytes are removed from the lateral compartment with the purpose of defining the true condylar anatomy to allow proper prosthesis placement. The improved visualization of the knee joint also allows better appreciation of the condylar anatomy and the relationship to the other compartments, thereby facilitating prosthesis placement12 (Fig. 7).
The distal end of the femur is first cleared of residual cartilage with either a scalpel or an oscillating saw (Figs. 8-A and 8-B). This maneuver is performed to avoid underresection of bone, which hinders recession of the leading edge of the femoral component (Fig. 9-A). This step is important in order to avoid impingement between the leading proximal edge of the femoral component and the patella. Precise identification of the junction of the cut surface of the distal femoral condyle and the overlying cartilage is critical in determining the placement of the anterior extent of the femoral component (Fig. 9-B). In medial unicompartmental knee arthroplasty, the prosthesis can be placed so that it abuts this junction. In lateral unicompartmental knee arthroplasty, however, the anterior edge of the femoral prosthesis should be 1 to 2 mm below the line. This difference is due to the laterally based tracking of the patella with knee motion and the greater tendency for patella-prosthesis impingement on the lateral side of the femur. For this reason, to err toward undersizing the anteroposterior dimension of the femoral component is preferable in order to avoid patellar impingement (Fig. 9-C). When downsizing the femoral component, however, one should be sure that metal-to-plastic contact is maintained in full extension (Fig. 10).
The femoral cutting guide is placed flush against the prepared distal femoral surface. The guide is placed parallel to the long axis of the femoral condyle. Removal of femoral osteophytes and complete visualization of the anatomy assist in proper placement. As an additional guide to prosthesis rotation, the planned posterior condylar resection should also be parallel to the expected tibial plateau resection. A spacer block can be placed between the femoral cutting jig and the tibia to assist in this process, if the tibial cut is made first (Fig. 11).
The periphery of the lateral plateau extends beyond the lateral border of the femoral condyle. Therefore, placement of the femoral and tibial prostheses requires careful positioning to ensure congruency (Fig. 12). The femoral component is displaced laterally in the medial-lateral dimension in order to maximize tibiofemoral component congruency in extension (Fig. 13).
An extramedullary tibial alignment guide is placed parallel to the tibial mechanical axis. The amount of femoral resection dictates only a minimal initial tibial resection in order to avoid the need for a thicker tibial component (Fig. 14). Only a small amount of posterior tibial slope (<5°) is created in order to minimize posterior wear (Fig. 15). The guide is secured with one or two pins as close to the tibial midline as possible to minimize the risk of a postoperative tibial plateau fracture. A reciprocating saw is used to make a sagittal cut immediately adjacent and lateral to the cruciate ligaments to facilitate removal of the cut tibial plateau. A rectangular rasp is used as needed to freshen the cut edges. The meniscus is removed carefully with electrocautery. Flexion and extension gaps should be equal and are evaluated with spacer blocks, and the tibial alignment is checked with an alignment rod. Sclerotic bone can be drilled to enhance cement interdigitation.
Preparation for prosthesis implantation begins with thorough pulsatile lavage followed by drying of the osseous surfaces. A single batch of cement is mixed at this time. The tibial prosthesis is cemented into place with use of a cement gun and finger pressurization. Excess cement on the prepared tibial surface is removed so that only a thin layer of well-pressurized cement remains (Fig. 16-A). The remainder of the cement is placed on the undersurface of the tibial prosthesis, which is deliberately inserted by impacting the component from posterior to anterior in order to displace excess cement anteriorly to facilitate removal (Fig. 16-B). Knee flexion and varus stress aid in femoral component placement, which is cemented with similar pressurization techniques with the remainder of the cement batch. The trial insert is placed, and the cement is allowed to harden with the knee in extension to pressurize the prosthesis-bone interfaces. All excess cement is thoroughly removed.
Postoperative rehabilitation protocols are initiated on postoperative day 1 with continuous passive motion. Patients are allowed to bear weight as tolerated with emphasis on achieving maximal extension and flexion. Patients are typically discharged from the hospital after two to three days to home or a rehabilitation facility as determined by the physical therapist and the patient's social situation. Patients are seen postoperatively at four weeks, one year, two years, five years, and then every two to three years for routine follow-up evaluation.
Noninflammatory unicompartmental arthritis
A patient deemed inappropriate for a realignment osteotomy
Preservation of the medial joint space radiographically
Symptoms isolated to the lateral side
Clinically correctable valgus alignment
Intraoperative inspection confirming arthritis predominantly isolated to the lateral compartment
Eburnated bone (Outerbridge Grade IV) in the patellofemoral compartment (Grades I, II, and III are accepted)13
Medial ligamentous laxity
An absent or deficient anterior cruciate ligament
A fixed flexion deformity of >10°
A fixed axial deformity of >10° from the neutral mechanical axis
Overresection of the tibial plateau necessitates use of a thick insert.
Failure to remove residual cartilage of the distal end of the femur leads to insufficient distal femoral resection.
Oversizing of the femoral component or inadequately recessing the leading edge of the femoral prosthesis causes patellar impingement.
When downsizing the femoral component, one should make sure there is sufficient metal-to-plastic contact when the knee is in full extension, or use the larger femoral prosthesis and/or recess the leading edge of the component.
Mismatch may occur during placement of the femoral and tibial components in the medial-lateral plane.
Improper rotation of the components may occur when they are placed without complete appreciation of the anatomical landmarks.
A lateral arthrotomy approach likely makes intraoperative abandonment to a total knee replacement unfamiliar and difficult.
Excessive posterior slope can encourage posterior wear patterns.
The described surgical technique has not changed substantially since the publication of the original paper. The frequency of lateral unicompartmental knee arthroplasties has remained constant throughout the career of the senior author.
The original scientific article in which the surgical technique was presented was published in JBJS Vol. 89-A, pp. 1948-54, September 2007
DISCLOSURE: The authors did not receive any outside funding or grants in support of their research for or preparation of this work. One or more of the authors, or a member of his or her immediate family, received, in any one year, payments or other benefits in excess of $10,000 or a commitment or agreement to provide such benefits from a commercial entity (DePuy Inc.). Also, a commercial entity (DePuy Inc.) paid or directed in any one year, or agreed to pay or direct, benefits in excess of $10,000 to a research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which one or more of the authors, or a member of his or her immediate family, is affiliated or associated.
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The line drawings in this article are the work of Joanne Haderer Müller of Haderer & Müller ().
Investigation performed at the Department of Orthopedic Surgery, Brigham and Women's Hospital, Boston, Massachusetts
- Copyright © 2008 by The Journal of Bone and Joint Surgery, Incorporated