Instability After Total Knee Arthroplasty

Jason H. Oh, MD

Giles R. Scuderi, MD, FACS

INTRODUCTION

Instability has been found to be the cause of 10% to 24% of total knee arthroplasty (TKA) revisions¹,²,³,⁴ and is the second most common cause of both early and late revisions, trailing behind infection in the early subgroup and aseptic loosening in the later subgroup.⁵,⁶ With the expansion of the aging population and increasing access to orthopedic care, the demand for TKA and subsequent revision arthroplasty has been increasing at an historic rate. With projections for primary TKA as high as 1.26 million to 1.68 million per year in the United States alone by 2030⁷ and annual increases in the numbers of revision TKA on the order of 13.5% annually,⁸ it is imperative for the surgeon to be able to effectively diagnose instability as the cause of TKA failure and then to make appropriate management decisions.

Upon initial patient evaluation, the clinician must bear in mind that the patient’s subjective complaint of “instability” may be completely unrelated to true mechanical instability. For example, buckling-type symptoms can commonly be attributed to pain or quadriceps weakness. Frank dislocation is rare, comprising only 0.5% to 3.3% of all revision TKA cases.⁹,¹⁰,¹¹ Patients with true instability are more likely to complain of anterior knee pain, multifocal areas of soft tissue tenderness, recurrent effusions, and difficulty walking or climbing steps.

Obtaining a comprehensive history is paramount to success in accurate diagnosis and management of the problem. Elements of the history that should be reviewed include the initial diagnosis that necessitated the primary arthroplasty procedure, prior knee injuries and surgeries, the presence of preoperative deformity and/or contractures, the date of the primary procedure, and the type of implants used. Ideally, the operative report should be obtained and carefully reviewed. Any history of connective tissue disorder or other risk factors for generalized ligamentous laxity should be elucidated. It is important to establish whether the arthroplasty was initially satisfactory and then went on to develop instability symptoms or whether the knee was persistently problematic from the initial procedure. The patient should be asked about any significant fluctuations in weight since the procedure, especially if there was a dramatic weight loss following bariatric surgery.

Upon physical examination, the patient’s gait should be carefully observed with attention to gait on both level ground, as well as while ascending and descending steps. Reliance on ambulatory aids should be noted. Muscular weakness should be graded, especially if associated with observable muscular atrophy. The integrity of the extensor mechanism should be confirmed. Stability testing for varus/valgus and anterior/posterior (AP) laxity should be performed at full extension, 30°, and 90°. The total range of motion should be recorded, with special attention to hyperextension or above-average hyperflexion. If an effusion is present, consider aspiration since a sterile hemarthrosis is a common finding associated with instability, with an average synovial red blood cell count of 65,000 per cubic millimeter found in one study.¹²

Full-length AP weight-bearing radiographs including the hip, knee, and ankle are helpful to establish the mechanical alignment of the limb. High-quality lateral views are important to accurately assess the tibial slope and femoral posterior condylar offset. Axial patellofemoral views (e.g., sunrise or Merchant views) should be obtained to assess patellofemoral alignment. The sizing and positioning of the components should be scrutinized, as should any evidence of component wear or breakage. Manual stress radiographs may be considered to gain additional information. Three-dimensional computed tomography imaging can be considered for cases of suspected subtle component malrotation or in preparation for revision surgery with significant bone loss. In all cases, infection must be ruled out prior to undertaking operative intervention.

Surgical treatment is warranted for most cases of symptomatic instability. The specific intervention should be tailored to the mechanism of failure, with the goal of achieving a stable knee with the level of constraint dependent upon the integrity of the collateral ligaments. Constrained implants (Fig. 58-1) have intrinsic stability because the enhanced tibial post provides varus-valgus stability. For cases with severe incompetence of the medial and lateral collateral ligaments, a rotating hinge knee prosthesis may be indicated (Fig. 58-2). While various methods of ligament repair and reconstruction have been described for ligament instability, we have found that good outcomes can be predictably attained with the use of constrained implants that substitute for the loss of ligament integrity (Table 58-1). Implantation of a constrained
prosthesis or rotating hinge should follow the principles of revision TKA, including the use of modular augmentation to address bone defects and stem fixation to transfer loads away from the prosthesis-bone interface to the diaphysis.

FIGURE 58-1 Intraoperative photograph (A) and anteroposterior radiograph (B) of the Zimmer CCK prosthesis (Zimmer Biomet, Warsaw, IN), a type of constrained prosthesis.

TIBIOFEMORAL INSTABILITY

Flexion Instability

Flexion instability is characterized by an inappropriately large flexion space that leads to painful episodes of tibiofemoral subluxation or frank dislocation (Fig. 58-3) while in flexion. Flexion instability is a risk factor for accelerated wear of the polyethylene insert and recurrent effusions. Symmetric flexion instability is associated with a rectangular flexion gap while asymmetric flexion instability is associated with a trapezoidal flexion gap. The diagnosis of flexion instability may be challenging because the main symptom is pain and radiographs are likely to demonstrate well-fixed, reasonably positioned components on weight-bearing views. However, careful attention while taking the history and physical examination can help elucidate the diagnosis of flexion instability, as well as the subtype and underlying etiology. Symmetric flexion instability is more common than asymmetric flexion instability. In general, symmetrical flexion instability is due to choosing a tibial articular surface that fills the extension gap, but it is too thin to fill the flexion gap. This is usually due to overresection of the posterior condyles with failure to restore the posterior condylar offset or underresection of the distal femur.

Asymmetrical flexion instability is characterized by a trapezoidal flexion gap. This condition may arise from inappropriate femoral component rotation (i.e., deviation from anatomic landmarks such as the epicondylar axis or femoral AP axis), isolated collateral ligament attenuation, or iatrogenic collateral ligament injury. The clinical ramifications of asymmetric flexion instability have been less well-studied than those of symmetric flexion instability; however, reports
have demonstrated improved range of motion and less pain with a rectangular flexion gap as opposed to a trapezoidal flexion gap.¹³

FIGURE 58-2 Intraoperative photograph of a rotating hinge prosthesis.

TABLE 58-1 Constraint—When Is It Needed?

	3 Ligament Knee	2 Ligament Knee		1 Ligament Knee		0 Ligament Knee
	CR	UC	PS	VVC	VVC	Hinged
Posterior cruciate ligament	X	–	–	–	–	–
Lateral collateral ligament	X	X	X	X	–	–
Medial collateral ligament	X	X	X	–	X	–
CR, posterior cruciate ligament-retaining; PS, posterior cruciate ligament-substituting; UC, ultracongruent; VVC, varus-valgus constrained.

Etiology

As mentioned above, symmetric flexion instability can arise either from surgical errors or from postoperative complications. Surgical errors in femoral preparation can be subdivided into failure to restore the femoral posterior condylar offset, overstuffing of the distal condylar offset, or a combination of the two. Similarly, symmetric flexion instability may be created on the tibial side by resecting the tibia with excessive posterior slope (Fig. 58-4). In addition, when cruciate-retaining implants are implanted, flexion instability may occur if the posterior cruciate ligament (PCL) is inadvertently damaged intraoperatively. A cruciate-retaining TKA that is well-balanced intraoperatively can still develop flexion instability postoperatively from PCL attenuation or rupture, which may be due to a trauma (e.g., a fall directly onto the flexed knee) or chronic attritional injury. Undiagnosed or subclinical rheumatic disease may play a role in chronic attritional pathology. PCL-substituting (PS) implant designs are protected against posterior tibial subluxation by the cam-post mechanism; however, even PS implants may be prone to painful and debilitating anterior subluxation in flexion, as well as accelerated wear of the polyethylene post, if left inappropriately balanced.

FIGURE 58-3 Lateral radiograph demonstrating posterior tibiofemoral dislocation in a total knee arthroplasty with flexion instability.

FIGURE 58-4 Lateral radiograph of a total knee arthroplasty with excessive tibial slope.

Late instability may be the result of gradual posterior, usually posteromedial, polyethylene wear. Radiographic review may reveal thinning of the tibial polyethylene insert, approaching proximity of the femoral and tibial components or subtle osteolysis.

Evaluation

In the initial evaluation of an unsatisfied patient with a TKA, it is important to maintain a high index of suspicion for flexion instability because the symptoms may be vague and subtle. The chief complaint is usually anterior
knee pain, including retinacular pain and/or pain at the tibial insertion of the pes anserinus tendons.¹⁴ Other common complaints include buckling, recurrent knee effusions, difficulty rising from a seated position, and difficulty managing stairs, especially on stair descent. Patients may also report that they feel dependent upon ambulation aids because of a sensation that they cannot trust the knee from “slipping out” underneath them.

On the physical examination, the patient’s gait must be observed both on a level surface and contrasted against the gait ascending and descending steps. Excessive caution when rising from a seated position should be noted. An effusion may be present; if so, aspiration and synovial fluid analysis can be considered to assess for the presence of hemarthrosis, as well as to rule out infection. Patients with isolated flexion instability will usually be stable to manual varus and valgus stress applied in full extension and 30° of flexion. AP drawer testing performed at 90° of flexion, with the leg in a dependent position hanging unsupported off the examination table, may demonstrate excessive translation. The absolute threshold of anteroposterior translation that may signify pathologic laxity will vary somewhat between different patients and different implant designs, though greater than 5 mm of translation has been identified as abnormal in some studies. The posterior sag sign noted at 90° is indicative of flexion instability in cruciate-retaining knees. In such cases, the quadriceps active test should be performed as an adjunct. Dial testing for posterolateral rotatory instability should be performed to assess the competency of the posterolateral corner structures. Lateral radiographs should be carefully assessed for posterior tibial subluxation. Posterior condylar offsets may also be measured and compared to the contralateral side.

Treatment and Outcomes

Conservative treatment for symptomatic flexion instability has limited indications. For the first instance of acute postoperative dislocation of a PS implant, closed reduction and bracing with adjunctive quadriceps strengthening has been described with maintenance of reduction in approximately 60% to 75% of cases.¹⁰,¹⁵,¹⁶,¹⁷ In cases of chronic instability without dislocation, a trial of quadriceps strengthening with or without bracing, with the use of symptomatic treatment for focal pain and swelling, may be used.

However, the mainstay of treatment for most cases of recurrent and symptomatic flexion instability should be surgical. While it may be appealing to revise the tibial polyethylene to a thicker size in order to stabilize the flexion space, if the femoral component is already appropriately sized, then the extension gap will become too tight and a flexion contracture may ensue. Careful intraoperative assessment must be performed to ensure flexion/extension space balancing and symmetry in order to prevent the recurrence of the same issue. If the cause of instability is determined to be an undersized femoral component, then the femur should be revised to an appropriately larger implant with the use of posterior condylar augments to restore the posterior condylar offset (Figs. 58-5 and 58-6). Alternatively, if an appropriately sized femoral component was placed upon an inadequate distal resection, the femur should be revised with proximal migration of the same sized component, thereby increasing the extension gap to match the flexion gap. A thicker tibial polyethylene insert should then be inserted. In both cases, the joint line should be restored to its anatomic location following the revision procedure.

If PCL rupture or attenuation is diagnosed as the root cause of flexion instability with a cruciate retaining implant, the knee should be revised to an ultracongruent or PS design. In cases of excessive tibial posterior slope, the tibia should recut with an appropriate degree of slope. The PCL insertion may be compromised in these cases and the use of a PS implant should be strongly considered.

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