Keith R. Berend, Jeffrey W. Salin, and Adolph V. Lombardi Jr.
DEFINITION
The knee classically has been divided into three compartments. Unicondylar knee arthroplasty (UKA) is performed for medial or lateral compartment arthritis.
Historically, patient selection, surgical technique, and component designs have been less than ideal. The key to a successful outcome for UKA is a successful marriage between these three variables.
ANATOMY
The three compartments of the knee are the lateral compartment, consisting of the lateral tibial plateau and lateral femoral condyle; the medial compartment, made up of the medial tibial plateau and medial femoral condyle; and the patellofemoral joint.
In the normal knee, most of the ligaments are at their resting, unstretched lengths in extension.
At about 20 to 30 degrees of flexion, the posterior capsule and lateral collateral ligament (LCL) slacken, allowing for a gap under tension.
Further gapping is resisted by the anterior cruciate ligament (ACL), posterior cruciate ligament (PCL), and medial collateral ligament (MCL).
At 90 degrees of flexion, the lateral compartment will distract 7 mm while the medial compartment maintains a constant 2-mm gap.
The cruciate ligaments and the MCL exert an isometric effect on the medial compartment throughout the range of motion.
In a single-leg stance, the load across the medial compartment is approximately 70%. This increases to 90% when there is a varus deformity of 4 to 6 degrees.
PATHOGENESIS
Medial unicompartmental osteoarthritis (OA) with intact cruciate ligaments and a functionally normal MCL results in a recognizable pattern of wear.
Cartilage and bone erosions are found on the anteromedial tibial plateau and distal surface of the femur, representing a pattern of extension disease.
Erosions rarely extend to the posterior quarter and never to the posterior joint margin of the tibial plateau.
Cartilage is preserved on the flexion surface of the femur and posterior tibia.
The intact ligaments maintain normal femoral “rollback,” resulting in this typical pattern of wear.
With a varus deformity, the posterior capsule shortens. However, when the capsule is relaxed at 20 degrees of flexion, the knee can be corrected manually to its prediseased alignment.
At 90 degrees of flexion, the knee corrects spontaneously as the cartilage on the flexion surface of the femur comes in contact with the posterior tibia.
The MCL subsequently is tensioned to its normal length in flexion.
Unlike tricompartmental disease, unicompartmental disease should not require any ligamentous release during arthroplasty.
To expose bone on both the tibia and femur, almost 5 mm of cartilage is lost. This typically causes 5 degrees of varus deformity.
Each additional millimeter of bone lost will result in increasing varus deformity of 1 degree.
NATURAL HISTORY
Degenerative failure of the ACL may be the event that causes anteromedial OA to make the transition to posteromedial OA, resulting in posterior subluxation and structural shortening of the MCL.
Varus deformity also will be maintained in flexion as the posterior cartilage is worn.
The ACL progresses through these stages of failure: normal, loss of synovial covering (usually distal), longitudinal splits in the exposed tendon, stretching and loss of strength, and, finally, rupture, with eventual disappearance of the ligament.
Chronic synovitis causes nutritional insufficiency and will place the ACL at risk.
Intercondylar osteophytes from arthritis also will put the ACL at risk of degenerative changes and failure.
PATIENT HISTORY AND PHYSICAL FINDINGS
Pain is the most common symptom of OA.
Pain usually is recognized along the medial joint line, but its localization is unreliable.
Pain is felt on standing and walking but usually is absent with sitting or lying down.
Varus deformity of 5 to 15 degrees while standing is seen. The deformity corrects with 90 degrees of flexion and upon valgus stress at 20 degrees of flexion.
Flexion contracture often is present, as are a joint effusion and synovial swelling.
The Lachman's, pivot shift, and drawer tests often are difficult to interpret in assessing the cruciate ligaments in an arthritic knee, but ligamentous stability should be present.
IMAGING AND OTHER DIAGNOSTIC STUDIES
Plain radiographs, including a weight-bearing anteroposterior (AP) and lateral (FIG 1A, B), stress, and patellar views, are critical in determining whether the patient is an appropriate candidate for UKA.
If only medial compartment arthritis (ie, joint space narrowing, sclerosis, cysts, osteophytes) is appreciated on the AP view, a valgus stress view (FIG 1C) is needed.
FIG 1 • A. Weight-bearing AP radiograph demonstrates medial joint space narrowing. B. Lateral radiograph shows preservation of the posterior cartilage. C. A valgus stress view radiograph demonstrates realignment of the ligaments and preservation of the joint space.
This will demonstrate the normal thickness of the cartilage in the lateral compartment and show whether the varus deformity is correctable.
The lateral compartment should not measure less than 5 mm (the sum of the thickness of the normal cartilage), and the medial compartment should gap at least 5 mm (the sum of the articular cartilage lost).
Incomplete loss of medial joint space must be investigated further with a varus stress view to show complete joint space loss.
If the joint space loss is not full-thickness, UKA should be avoided and other causes for the pain should be sought.
Lateral radiographs reveal the extent of posterior wear and are, therefore, a reliable indicator of the functional integrity of the ACL.
The femoral condyles and tibial plateau should appear superimposed.
If a concave erosion is present on the anterior two thirds of the plateau, the ACL is intact (95% probability).
DIFFERENTIAL DIAGNOSIS
Tricompartmental OA
Meniscal tear
Saphenous neuritis
Osteochondral injury
Pes anserine bursitis
Septic arthritis
NONOPERATIVE MANAGEMENT
Nonoperative management consists of nonsteroidal antiinflammatory drugs, glucosamine and chondroitin, physical therapy, assistive devices, viscosupplementation, bracing, or intra-articular corticosteroid injections.
SURGICAL MANAGEMENT
The patient's suitability for UKA ultimately is decided in the operating room. The patient must have been prepared in advance for a possible conversion to a total knee arthroplasty if not all the criteria are met.
We use the Oxford criteria, as follows:
Physical signs must include pain severe enough to justify joint replacement and flexion deformity less than 15 degrees.
Radiographic signs include full-thickness cartilage loss with eburnated bone-on-bone contact in the medial compartment; full-thickness cartilage preservation in the lateral compartment; intact articular surface at the back of the tibial plateau; and manually correctable varus deformity.
Intraoperative signs include the presence of an intact ACL and satisfactory appearance of the central articular cartilage of the lateral compartment.
Traditional contraindications for UKA include:
Inflammatory arthritis
A flexion contracture of 5 or more degrees
A preoperative arc of motion of less than 90 degrees
Angular deformity of more than 15 degrees
Significant cartilaginous damage in the opposite compartment
ACL deficiency
Body mass index higher than 32
Exposed subchondral bone beneath the patella
Criteria that are not considered contraindications are age, activity level, weight, patellofemoral arthritis, degenerative lateral meniscus, and chondrocalcinosis.
Preoperative Planning
Preoperative templating for the appropriately sized components is performed using the lateral radiograph.
FIG 2 • A patient is prepared for unicompartmental knee arthroplasty positioned in the hanging leg position.
Examination is performed under anesthesia to assess the stability and motion of the knee.
Positioning
A thigh tourniquet is applied and the leg is placed in a hanging leg holder (FIG 2).
FIG 3 • Photograph of the flexed knee shows anatomic landmarks referenced during the surgical approach.
The hip is flexed to about 30 degrees, and the leg is abducted.
The knee must be free to flex to at least 135 degrees.
Approach
An abbreviated medial parapatellar approach is used for a medial UKA (FIG 3).
TECHNIQUES
Exposure
With the knee flexed to 90 degrees, a medial parapatellar skin incision is made from the medial margin of the patella to a point 3 cm distal to the joint line. The capsular incision is extended obliquely and medially for 1 to 2 cm into the vastus medialis.
Part of the retropatellar fat pad is excised, and the anterior cruciate ligament is inspected to ascertain that it is intact (TECH FIG 1A).
All osteophytes must be removed from the medial margin of the medial femoral condyle and from both margins of the intercondylar notch (TECH FIG 1B).
TECH FIG 1 • A. An abbreviated medial parapatellar surgical approach is used. B. An osteotome is used to remove osteophytes from the margins of the femoral condyles and intercondylar notch.
TIBIAL RESECTION
The front of the tibia is exposed in the lower part of the wound from the tibial tubercle to the rim of the plateau.
As much of the medial meniscus is excised as possible.
None of the fibers of the medial collateral ligament are released.
The tibial saw guide is applied with its shaft parallel with the long axis of the tibia in both planes (TECH FIG 2A).
The level of resection is estimated—this level varies according to the depth of tibial erosion.
A minimum of 6 mm of intact posterior cartilage and bone should be removed.
The saw cut should pass 2 or 3 mm below the deepest part of the erosion.
It is better to be conservative with the first cut, because the tibia can easily be re-cut if too little bone has been removed (TECH FIG 2B).
TECH FIG 2 • A. The tibial alignment guide is placed parallel to the tibia in both the AP and lateral planes. B. With the tibial alignment guide in place, it is possible to observe the depth of tibial bone to be resected. C. A reciprocating saw is used to make the vertical tibial cut. D. A 12- mm-wide oscillating saw is used to excise the tibial plateau.
A reciprocating saw with a stiff narrow blade is used to make the vertical tibial cut.
The blade is pushed into the intercondylar notch close to the lateral margin of the medial femoral condyle.
The blade is pointed toward the head of the femur (TECH FIG 2C).
Before the horizontal cut is made, a retractor is inserted to protect the medial collateral ligament.
A 12-mm-wide oscillating saw blade is used to excise the tibial plateau (TECH FIG 2D).
The blade must go right to the back of the joint.
When the plateau is loose, it is levered up with a broad osteotome and removed.
The posterior horn of the medial meniscus can now be removed.
The excised plateau will demonstrate the classic lesion of anteromedial OA and preserved cartilage posteriorly.
The excised plateau together with the tibial templates is used to choose the size of the tibial implant.
TEMPLATING AND DRILLING
Sufficient thickness of bone must be removed from the tibia to accommodate the tibial template and a bearing at least 4 mm thick (TECH FIG 3A–C).
Whenever a feeler gauge is used to measure a gap, the retractors are removed.
If the retractors are left in, they have the effect of tightening the soft tissues, artificially diminishing the gap.
If the 4-mm gauge cannot be inserted or feels tight, then more bone needs to be excised from the tibia.
With the knee in about 45 degrees of flexion, a hole is made into the intramedullary canal of the femur with the awl. The hole must be situated 1 cm anterior to the anteromedial corner of the intercondylar notch.
The knee is then flexed to 90 degrees. This must be done with care, because the medial border of the patella abuts against the rod.
The tibial template is replaced, the femoral drill guide is inserted, and a feeler gauge is placed that is 1 mm thinner than the flexion space between them.
If the gauge is too loose, a thicker gauge is inserted.
The femoral drill guide is now manipulated until it is in the middle of the condyle and its handle is aligned parallel with the long axis of the tibia.
By adjusting the degree of flexion of the knee, the upper surface of the drill guide is made to lie parallel with the intramedullary rod when viewed from the side.
TECH FIG 3 • A. The resected fragment of tibial bone demonstrates anteromedial osteoarthritis and intact posterior cartilage. B. Tibial sizing templates are aligned on the resected tibial fragment to determine appropriate component size. C. Retractors are removed and a no. 4 feeler gauge is inserted to measure the gap between femur and tibia. D–F. With the tibial template and a feeler gauge 1 mm thinner than the flexion gap in place, the femoral drill guide is inserted and positioned to determine femoral alignment. When all alignment requirements are fulfilled, holes are drilled.
By internally and externally rotating the tibia, the lateral surface of the fin is made to lie parallel with the intramedullary rod when viewed from above (TECH FIG 3D).
When all of these five requirements are fulfilled, the drill is passed through the upper hole to its stop and left in place.
The other hole is then drilled, and both drills and all instruments are removed from the joint.
The intramedullary rod also may be removed (TECH FIG 3E,F).
POSTERIOR CONDYLE RESECTION AND MILLING
The femoral saw block is inserted into the drilled holes.
Using the 12-mm broad sagittal saw, the posterior facet of the femoral condyle is excised (TECH FIG 4A).
The saw block is removed, and the 0 spigot is inserted into the large drill hole (TECH FIG 4B).
The spherical cutter is then used to mill the distal femur until the cutter will not advance further (TECH FIG 4C).
With the leg in 90 degrees of flexion, the tibial template is inserted and the femoral trial component is applied to the milled condyle (TECH FIG 4D).
TECH FIG 4 • A. With the femoral saw block inserted into the drilled holes, a 12mm broad sagittal saw is used to resect the posterior facet of the femoral condyle. B. Upon removal of the femoral saw block, a 0 spigot is inserted into the large drill hole. C. A spherical cutter is used to mill the distal femur. D. With the knee flexed to 90 degrees, the tibial template is placed, and the femoral trial is applied to the milled distal femoral condyle.
BALANCING THE FLEXION AND EXTENSION GAPS
The flexion gap is now carefully measured with feeler gauges. The feeler gauge will slide in and out easily but will not tilt.
The gauge is removed. It is important to remove the gauge before extending the knee because, at this stage, the extension gap is always narrower than the flexion gap.
If the gauge is left in place, it may stretch or rupture the ligaments as the knee extends.
Next, the extension gap is measured in 20 degrees of flexion with the metal feeler gauges (TECH FIG 5).
In full extension, the posterior capsule is tight, and its influence gives a false under-measurement.
TECH FIG 5 • The extension gap is measured with the knee in 20 degrees of flexion using metal feeler gauges.
The formula for balancing the flexion and extension gaps is as follows:
Flexion gap (mm) − extension gap (mm) = thickness of bone to be milled from femur (mm) = spigot number to be used.
For instance, if the flexion gap measures 5 mm and the extension gap 2 mm, the amount of bone to be milled is 3 mm. To achieve this, a number 3 spigot is inserted and the bone is milled until the cutter will advance no further.
With the tibial template and the femoral trial component in place, the flexion and extension gaps are remeasured.
TRIAL COMPONENT AND TRIAL BEARING
The tibial trial component is inserted and tapped home with the tibial impactor.
The trial component must be flush to the bone, and its posterior margin must extend to the back of the tibia.
Only a light hammer should be used, to avoid the risk of plateau fracture.
Final preparation of the femur requires trimming of the condyle anteriorly and posteriorly to reduce the risk of impingement of bone against the bearing in full extension and full flexion (TECH FIG 6A).
The femoral posterior trimming guide is applied to the condyle, and the osteophyte chisel is used to remove any posterior osteophytes.
A trial meniscal bearing of the chosen thickness is inserted.
It is only at this stage that a trial bearing is used (TECH FIG 6B–D).
Previously, feeler gauges have been used to measure the gaps, because they do not stretch the ligaments.
With the bearing in place, the knee is manipulated through a full range of motion to demonstrate stability of the joint, security of the bearing, and absence of impingement.
The thickness of the bearing should be such as to restore the ligaments to their natural tension so that when a valgus force is applied to the knee, the artificial joint surfaces distract a millimeter or two.
This test should be done with the knee in 20 degrees of flexion.
In full extension, the bearing will be gripped firmly because of the tight posterior capsule.
TECH FIG 6 • A. The femoral posterior trimming guide, osteophyte chisel, and femoral trial are shown. B. A meniscal bearing trial, shown before insertion. C. Tension in extension is checked using the meniscal bearing trial with the knee in 15 degrees of flexion. D. Tension is then checked with the knee in full flexion.
INSERTING AND CEMENTING THE FINAL COMPONENT
The femoral and tibial surfaces are roughened by multiple small drill holes made with the cement key drill (TECH FIG 7A).
The tibial component is inserted and pressed down, first posteriorly and then anteriorly, so that excess cement is squeezed out at the front (TECH FIG 7B).
Excess cement is removed from the margins of the component with a small curette (TECH FIG 7C).
The femoral component is applied to the condyle and impacted with the punch held at 30 degrees to the long axis of the femur.
Again, excess cement is removed from the margins with a small curette.
During cement setting, the leg is held in 45 degrees of flexion (TECH FIG 7D).
The leg should not be fully extended, because pressure in this position may tilt the tibial component anteriorly.
The reconstruction is completed by snapping the chosen bearing into place.
Routine closure of the wound follows.
TECH FIG 7 • A. The femoral and tibial surfaces are roughened to enhance cement interdigitation, and the soft tissues are injected with a mixture of ropivacaine, ketorolac, and epinephrine as part of the multimodal pain management protocol. B. The tibial component is inserted and pressed down using specialized instrumentation, first posteriorly and then anteriorly, so that excess cement is squeezed out at the front. C. Excess cement as shown may be removed using a small curette. D. While the cement is setting, the leg is maintained in 45 degrees of flexion.
POSTOPERATIVE CARE
Intraoperative local anesthetic (60 mL of 0.5% ropivacaine with 30 mg ketorolac and 0.5 mL of 1:1000 epinephrine) is injected into the damaged soft tissues.
A drain is used; it is pulled on the first postoperative day.
Patients typically are discharged from the hospital less than 24 hours after admission.
Patients usually have a rapid initial recovery (FIG 4). Some patients may experience mild medial pain for 2 to 3 months after surgery.
OUTCOMES
UKA is a predictable surgical option for appropriate candidates with anteromedial OA. Survival rate of UKA at 10 years is 90% or better (Table 1).
FIG 4 • Patients typically enjoy rapid recovery, achieving high flexion with normal kinematics.
We performed 316 medial Oxford UKA between 2004 and 2005. Classic contraindications to UKA, such as obesity, young age, patellofemoral degenerative joint disease (DJD), and anterior knee pain, were specifically reviewed.
Twenty-five percent of patients were obese, with BMI greater than 35.
Fifty-four percent of patients were younger than 60 years of age.
The incidence of radiographic patellofemoral degenerative disease was 43% preoperatively.
Only 68% of patients reported isolated medial-sided knee pain preoperatively. Twenty-one percent described global knee pain and 6% reported anterior knee pain.
There were no differences in failure rates or Knee Society scores between patients with preoperative anterior knee pain and those without anterior knee pain before UKA. No differences in outcomes were noted between those with and those without radiographic evidence of patellar DJD.
No differences in outcomes were noted between obese and non-obese patients. Additionally, no differences in outcomes were seen between patients younger or older than 60 years of age.
Our experience with the Oxford UKA now spans 2.5 years. To date, our patients have experienced five implant failures: two for tibial loosening with collapse, one for tibial plateau fracture, one for infection, and one for unexplained pain.
There have been no dislocations of the mobile bearing.
FIG 5 • Although the meniscal bearing trial is universal, the actual bearing is anatomic, with longer lateral wings to enhance stability.
COMPLICATIONS
Multiple series (Table 1; see also references 9 and 10) provide an overview of the potential complications and outcomes of the Oxford Mobile Bearing UKA.
Infection has been observed in up to 0.6% of cases.
Medial tibial plateau fracture occurs in 0.3% of cases. Care must be taken not to damage the posterior cortex.
Retrieval studies of the Oxford UKA show wear to be 0.026 to 0.043 mm per year.
Historically, dislocation of the mobile bearing occurs in 0.5% of cases. Primary dislocations occur for two main reasons: impingement and femoral component malposition. The bearing can impinge on posterior osteophytes or cement, causing the bearing to “spit out” anteriorly. If the femoral component is not positioned correctly, the mobile bearing can “spin out.” The newer bearing with “wings” will help resolve some spin-out issues (FIG 5). However, specific attention to placement of the femoral component is crucial.
Progression of lateral compartment arthritis requiring revision occurred in 1.4% of patients with UKAs. Most authors believe that progression of lateral compartment OA is due to overcorrection of a varus deformity to valgus due to MCL release or damage.
As many as 2% of patients may have persistent unexplained pain, possibly caused by tibial condyle overload, overhang of the tibial component, overstretching of the MCL, and pes anserine bursitis by 1 year post-surgery; most medial pain resolves spontaneously by 2 years.
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