Total knee arthroplasty (TKA) is a reliable and successful procedure with survivorship greater than 92 percent at 10 years. Although only a small percentage of primary TKAs require revision surgery, the absolute number of these patients is considerable.

Nearly 720,000 primary TKA procedures were performed in the United States in 2010. In 2011, based on data from the HCUP Nationwide Inpatient Sample, 2006-2011, nearly 70,000 TKA revisions procedures were performed.

The most common modes of early TKA failure (within the first 5 years after surgery) are infection and instability in near equal numbers. Instability is the diagnosis in 10 percent to 22 percent of late revision TKAs, which makes it a leading cause for revision. An understanding of the etiology and diagnosis of the unstable TKA is critical to appropriate treatment.

Evaluation
A thorough history and physical examination of the knee can be helpful in determining the etiology of TKA instability. Vague pain, effusions, apprehension, or difficulty with stairs are clues that suggest instability. However, an effusion, hemarthrosis, and recurrent synovitis are not specific to any single type of instability.

The examiner should check varus-valgus stress of the collateral ligaments in extension, at 30 degrees of flexion, and at 90 degrees of flexion. A drawer test at 90 degrees of flexion evaluates the anterior-posterior stability of the prosthesis. A positive sag sign or quadriceps active test indicates posterior cruciate ligament (PCL) insufficiency in a cruciate-retaining (CR) prosthesis. Excessive or recently increased flexion may also indicate PCL incompetence. Rarely, genu recurvatum may be a clear cause of instability and signify soft-tissue deficiency or neurologic impairment.

Patients with symptoms suggestive of instability require a thorough radiographic and laboratory evaluation to confirm the diagnosis. Objective evaluation includes a complete series of radiographs of the affected joint, including long-leg radiographs to rule out prosthetic loosening and malalignment. In some cases, a CT scan is helpful to evaluate rotational alignment of the femoral and tibial components, and nuclear imaging is a useful tool to rule out mechanical loosening of the prosthesis.

Laboratory tests, including C-reactive protein and erythrocyte sedimentation rate, must be performed to rule out periprosthetic infection. Analysis of the synovial fluid, including aspiration with cell count and culture, is important to rule out infection.

Classification and management strategies
Instability is generally described based on the evocative position in which it is experienced by the patient (flexion or extension instability). Conversely, the direction of instability may be used for classification (eg, coronal plane, sagittal plane, or global instability).

One analysis of a revision arthroplasty population identified the following six categories for postarthroplasty instability:

• flexion/extension gap mismatch
• component malposition
• isolated ligament insufficiency
• extensor mechanism insufficiency
• component loosening
• global instability

Overlap may exist because the reason for an unstable TKA may be multifactorial. Regardless of the classification, identification of the source(s) of the instability is essential for successful treatment.

Extension instability typically arises from problems with extension gap balancing. Excessive distal femoral resection will lead to isolated extension instability that cannot be managed with a large polyethylene insert. This problem can be managed with the use of distal femoral augments to decrease the extension gap while maintaining the flexion gap.

Excessive tibial resection increases both the extension and the flexion gaps equally. The use of a thicker polyethylene insert may fill the gaps, but this may also lead to patellar impingement as well as increased stress on the tibial component over time. Excessive medial or lateral soft-tissue release also can lead to varus-valgus instability in extension that can be treated with increased implant constraint with or without soft-tissue reconstruction.

Flexion instability occurs when the flexion gap is significantly larger than the extension gap. Excessive or inaccurate resection of the posterior femoral condyles will lead to flexion-extension gap mismatch. An improperly sized femoral component may lead to asymmetry between the flexion and extension spaces. Insufficient distal femoral resection with an appropriate posterior femoral resection leads to a tight extension space. If the tibial cut is adjusted to create a normal extension gap, then the flexion space will be inappropriately large.

An intraoperative positive anterior drawer of more than 5 mm with the patella reduced in the trochlea may indicate flexion instability. Filling the flexion gap with a thicker polyethylene may provide increased flexion stability, but a corresponding tightness in the extension gap could result in a flexion contracture. Posterior femoral augments and/or an upsized femoral component are the appropriate initial correction in flexion instability.

Tibial slope can be a subtle determinant of flexion stability, since excessive or inadequate slope can create difficulty in balancing the flexion and extension gaps. For example, with insufficient tibial slope (usually < 3 degrees) a corresponding recession of the PCL may be required in order to achieve flexion gap balance in a CR prosthesis. Over time, progressive attenuation of the PCL may lead to symptomatic anterior-posterior instability. In this situation, most patients will require revision to a posterior-stabilized (PS) prosthesis.

Although it may be possible to use a thicker CR polyethylene insert, the reported long-term success of this procedure alone is low. Anterior-constrained CR polyethylene inserts are available, but they may not be an adequate long-term solution to PCL-deficient knees that are loose in flexion. In cases of excessive tibial slope, conversion to a PS construct may not be sufficient to correct a flexion gap imbalance, and revision of the tibial component to correct the slope may be required.

Late TKA instability (after 5 years postoperatively) is less common than early instability. Polyethylene wear, component loosening, and ligament or extensor mechanism attenuation are some of the potential causes. Polyethylene wear is often caused by component malalignment. Appropriate component alignment should be verified prior to polyethylene exchange for wear. Long-term results of isolated polyethylene exchange have been associated with early failures in some cases.

Patient factors—such as connective tissue disorders or inflammatory arthritis—may contribute to instability, especially with a CR prosthesis. Patients who have neuromuscular diseases, quadriceps or hip abductor weakness, or neuropathic joints should be considered for primary constrained prostheses, such as a constrained condylar or a rotating hinge implant. Obese patients have an increased risk of collateral ligament compromise due to difficult exposure.

Midflexion instability
Midflexion instability is becoming a more widely accepted concept. It is characterized by a knee that is stable both in full extension and in flexion at 90 degrees, but instability develops during the arc of motion from 0 degrees to 90 degrees. Loss of ligamentous isometry due to component malposition or over-release of a portion of the medial collateral ligament may be responsible for this phenomenon.

Midflexion instability was initially thought to be due to multiradius components allowing transient relaxation of the collateral ligaments during knee flexion. However, this idea was not supported by a cadaveric study of multiradius and single-radius femoral component designs.

A recent fluoroscopic study examining the translation kinematics during knee flexion of two groups of post-TKA patients—those with high postoperative flexion and those with lower flexion—found that patients with limited flexion had an increased incidence of femoral condylar liftoff during flexion. The results suggested that patients with high maximum flexion have greater midflexion stability. A recent computer navigation study demonstrated that varus-valgus instability in midflexion may be greater with PS versus CR knee implants. Attempts to characterize the causes and treatment of midflexion instability are ongoing.

Basics of revision arthroplasty
At the time of revision, the initial correction should address the underlying diagnosis of instability. However, the minimal amount of implant constraint should be used to provide a balanced and stable construct. In most cases, the surgeon should be prepared to increase constraint. The indication to use a constrained prosthesis is an inability to achieve adequate soft-tissue balance and a stable range of motion. Similarly, a rotating hinge may be necessary in cases of significant tissue loss or anteroposterior instability.

Ligament advancement, repair, or reconstruction may be considerations for younger patients in whom a constrained implant may be undesirable. These procedures have had good outcomes in a number of small series, but they should take place in conjunction with component revision. Survivorship of 90 percent of constrained condylar implants at 10-year follow-up has been reported in both cementless and cemented designs. In the setting of severe bone loss or global instability, the use of a rotating hinged prosthesis has a reported survivorship of 68 percent at 5 years and a significantly increased rate of complications.

The diagnosis and subsequent treatment of the unstable TKA can be perplexing. A complete history, physical exam, labs, radiographs, and evaluation of alignment are important parts of the workup. When the appropriate surgery is matched to the underlying etiology, revision arthroplasty for the unstable knee may be successful.

Matthew A. Popa, MD, is a fellow in the department of orthopaedics at University Hospitals Case Medical Center; Glenn D. Wera, MD, is assistant professor, orthopaedics, Case Western Reserve University (CWRU) School of Medicine; and Victor M. Goldberg, MD, is professor of orthopaedics at CWRU.

Disclosure information: Drs. Popa and Wera—no conflicts; Dr. Goldberg— Osiris Therapeutics, Astrazenica, Ferring Pharmaceuticals, Elsevier, AAOS Now, Clinical Orthopaedics and Related Research, Journal of Arthroplasty, Austin Bioinnovation Institute, Arthritis Research Society International.

Bottom Line

• A thorough workup and evaluation is recommended prior to pursuing revision TKA due to instability.
• Surgeons should address the underlying cause of instability at the time of revision.
• Surgeons should be prepared to increase constraint at the time of surgery.
• Midflexion instability is emerging as an important topic in TKA.

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