Evaluation and treatment of flexion, midflexion, and global instability

Flexion instability

Flexion instability is caused when the flexion gap exceeds the extension gap [3]. This situation can occur, for example, in poorly executed TKA stabilized posteriorly; because these do not provide a varus-valgus constraint, sacrifice of the posterior cruciate ligament (PCL) can increase the flexion gap and lead to collateral ligament laxity [4].

In contrast to extension instability, where the knee can become unstable during activities such as walking [5], flexion instability occurs when the knee is bent and loaded and thus becomes symptomatic as the patient ascends or descends stairs, or transfers from sitting to standing [5]. Flexion instability is associated with a cluster of additional symptoms; aside from the instability when getting up from a sitting position or when using stairs, whereby a patient may feel anterior knee pain, the patient may have recurrent serosanguinous effusions [6] or synovitis and hemarthroses [3]; more than 60% of patients with flexion instability had a serosanguinous aspirate in one study [7]. Additionally, patients have tendinopathy at the pes anserinus and iliotibial band, periarticular bursitis of the knee [8], and on examination will test positive on the anterior drawer test [6]. In a study by Abdel et al [4], the clinical findings of patients with flexion instability were well described. Here, patients had substantial tibial translation at 90° of flexion of often more than 1 cm and effusion which was often hemarthrosis on aspiration [4]. Additionally, patients had soft-tissue tenderness in the area of the pes anserinus, peripatellar soft tissues, and hamstring tendons [4]. As Oussedik notes, the patient typically presents to the clinic complaining of pain somewhere in the arc of motion or when ascending or descending stairs, though often the point of pain is hard to isolate for the patient. The patient may also have recurrent effusions, swelling, impingement, or gross instability, which could result in a serosanguinous aspirate should this be examined for. When assessing the patient, certain clinical signs are present such as laxity in flexion. Oussedik comments that an examination under general anesthetic with an image intensifier is most interesting: fluoroscopy increases the diagnostic ability of the surgeon beyond the conventional clinical examination, allowing the surgeon to fully evaluate the knee, with instability being assessed in all planes and through the full range of motion [9]. Stress x-rays also offer quantifiable measurements of joint laxity.

In order to better understand the factors leading to instability in flexion, Abdel et al [4] assessed patients who had undergone revision TKA for flexion instability. When comparing the pre- and postoperative radiological comparisons of the 60 included patients, it was shown that there was a significant increase in mean posterior condylar offset, significant reduction in posterior slope, and significant reduction in the distance from the epicondylar axis to the femoral component, all of which were noted as factors leading to flexion instability [4]. It could therefore be deduced that in this patient group component malrotation, excessive posterior tibial slope, inappropriate condylar offset, and distalization of joint line contributed to flexion instability [4].

Stepwise technique for revision total knee arthroplasty in patients with flexion instability

When a revision TKA is indicated in patients with flexion instability, the goals are to restore alignment, rotation, and balance between the flexion and extension gaps [4]. One of the most commonly cited methods for revision TKA in these patients is the stepwise technique described by Abdel et al [4]. This technique starts by first addressing the excessive tibial slope, after which they correct the axial and rotational malalignment of the components. They then introduce a larger femoral component (by a mean of 4 mm) which increases the posterior condylar offset and diminishes the flexion gap. Abdel et al note that if the flexion gap still remains larger than the extension gap, an additional distal femoral resection may be needed. After this, a larger polyethylene insert is typically required [4, 11]. Oussedik cautions that an additional distal femoral resection can elevate the joint line, which in itself can lead to problems. Thus, it is important to carefully measure the joint line before performing an additional resection; if there is a mismatch despite a well-positioned joint line, Oussedik recommends increasing the constraint.

Increasing constrain in rerevisions for recurrent instability

In the series of patients included in the study by Abdel et al [4], approximately two-thirds of them had a primary cruciate-retaining TKA with the remainder having a posterior-stabilized TKA. For the revision TKA, all knees were revised to a posterior-stabilized design. At the 10-year postrevision TKA for flexion instability, only three of the patients available for follow-up underwent rerevision for recurrent flexion instability and one for global instability [8]. The treatment method of choice for a rerevision in those patients was to move to a more constrained implant in two patients, who received varus-valgus constrained implants. For the third patient, the strategy was to switch the posterior-stabilized polyethylene insert to a varus-valgus polyethylene insert. Similarly, for the patient with global instability, the strategy was to increase the thickness of the polyethylene insert and to increase the constraint by revising the posterior-stabilized TKA to a varus-valgus constrained implant. In a study by Hannon et al [12] on patients with flexion instability, rerevision was indicated in seven patients with recurrent instability and three with global instability. Of these patients, six were rerevised to exchange the polyethylene liner for a larger size after having received an original varus-valgus constrained implant on revision. Three patients had received a posterior-stabilized component on first revision, all three received implants with more constraint, two were varus-valgus constrained implants and one was a hinged implant. The last patient had first undergone revision to receive a larger polyethylene insert and was rerevised to a varus-valgus constrained implant.

Polyethylene insert exchange may be successful in carefully selected patients

Although a thicker insert will add to both flexion and extension gaps, this often results in poor and unpredictable results because it does not address the mismatch in the flexion and extension gaps [5]. There is also a risk of creating a flexion contracture in place of flexion instability [5]. Furthermore, there have been reports of an up to 50% 3-year failure rate of isolated polyethylene exchange (IPE) in revision TKA [5]—thus, it is not routinely recommended to perform such an IPE except where there is posteromedial polyethylene wear producing a pseudoinstability [5]. There has, however, been a report by Green et al [13] of successful IPE in carefully selected patients with flexion instability after TKA, who had excellent radiographic alignment and component positioning preoperatively, excellent intraoperative component rotation, sizing and fixation, and in whom the flexion and extension gaps could be balanced. In one cohort of patients undergoing IPE, the distal femoral angle in the majority of the patients was cut at approximately 3–6° of valgus with respect to the anatomical axis; additionally, in most patients the proximal tibial cut angle was within 1° of the neutral mechanical axis [14]. Most patients also had a posterior slope of 0–7° [14]. The authors thus noted that the patients had been carefully selected prior to IPE [14].

Evolving surgical techniques

Hannon et al [8] noted that the surgical techniques had evolved since the original study by Abdel et al [4]. In the original study all patients underwent revision TKA with a posterior-stabilized design. At the time of the 10-year follow-up, though, the authors routinely performed revision TKAs with implants with a higher level of constraint (eg, varus-valgus constrained or hinged implants when it was not possible to balance the extension and flexion gaps) [8]. Abdel and Haas [10] explain that the earlier posterior-stabilized designs were able to prevent dislocations because the jump distance over the stabilizing tibial peg was sufficient, but if the flexion gap was loose enough, it could allow substantial tibial translation. Furthermore, the standard posterior-stabilized designs lacked varus-valgus constraint, which meant that a loose flexion gap could lead to flexion instability [10]. As Hannon et al [8] note, using a higher level of constraint on revision TKA may mitigate the need for rerevision at a later stage; this is supported by Al-Jabri et al [6], who comment that there should be a low threshold to convert to a more constrained implant. A second study by Hannon et al [12] comparing nonoperative and surgical methods of treating flexion instability showed that good survivorship of 87% at 10 years was seen with revision TKA, which was consistent with other studies. However, instability was the most common reason for a rerevision, and it was noted that the functional improvements gained were modest [12]. Oussedik comments that despite the good survivorship observed with revision TKA for instability, patients need to be aware that there may still be slight reductions in range of motion or some functional limitations.

Midflexion instability

A recent review by Mehta and colleagues [2] attempted to define midflexion instability given that there has, until recently, been no standard definition available [5]. Midflexion instability, despite being difficult to define, is regarded as a distinct entity to flexion instability [15].

Symptoms of midflexion instability are difficulty in climbing stairs or getting out of a vehicle [4], as this occurs when the flexed knee is loaded [2], and patients can report not being fully confident with the knee [5] or may note a "subtle sense of laxity to an extreme sense of dislocation" [2]. Additionally and similar to flexion instability, patients may experience recurrent joint effusions, pain at the insertion sites of the tendons [2], and have anterior knee pain [5]. They may also be observed wearing a knee brace to provide additional stability [5]. Oussedik comments that in his clinical experience it is unusual that a patient complains of 'instability' per se, rather they talk about having pain and discomfort around the tibiofemoral joint when loading in flexion. This is a diagnostic clue as this pain is different to the anterior pain which would be experienced with patellofemoral complications. Patients may also have unexplained effusions, and clinical signs will include laxity. However, diagnosis remains difficult; instability is subjective, with some patients feeling that the knee is unstable at low levels of laxity with others felling unstable at higher levels. Diagnosis of midflexion instability is based on this series of clues, as there are no tests and the literature struggles to fully define this type of instability. Thus, as in flexion instability, performing a clinical examination under anesthesia can allow the surgeon to better quantify the laxity.

As noted above, the literature has struggled to provide a clear definition of midflexion instability. Definitions vary (Table 1) [2], with one report noting that to distinguish between flexion instability and midflexion instability, the latter occurs when the knee is stable in full extension and 90° of flexion but is unstable around halfway between the two (Table 1) [5]. Mehta et al [2] comment that midflexion instability is instability in flexion at > 0° but < 90° of flexion. One recommendation is to assess for varus-valgus instability at between 30° and 60° during assessments [5]. As Oussedik notes, different structures provide stability to the knee at different angles. In full extension to around 10° of flexion, the posterior capsule of the knee provides varus-valgus stability; at this point the posterior capsule should be taut. However, as the knee is flexed, the posterior capsule stabilizing the joint relaxes and there is an isolated effect of the collateral ligaments; thus, there needs to be more than 10° of flexion to get laxity in the posterior capsule. As the knee flexes further to 90°, the tibial component engages below the posterior condylar femoral component; if there is instability at 90°, this would be considered as flexion instability. Thus, in Oussedik's opinion, midflexion instability is between 30° and 60° of flexion, where at 30° of flexion the capsule is disengaged and at 60° the tibial component is not under the condyles of the femoral component. This definition supports the classical description of midflexion instability first described by Martin and Whiteside [16] and discussed by Mehta et al [2] in their review, by which midflexion instability is described as "laxity in the coronal plane (varus/valgus) between 30° and 60° of flexion".

The three main reported factors associated with midflexion instability are elevated joint line, multiradii femoral components and medial collateral ligament laxity (Table 2) [5]. The evidence supporting elevated joint line is mostly limited to cadaveric studies, yet there is evidence that if the joint line is elevated more than 8 mm during a primary TKA, it is associated with poor results [5]. Joint line elevation may occur when a large distal femoral cut is made to compensate for preoperative flexion contracture [2]. In the review by Mehta et al [2] on midflexion instability, the authors note the conflicting information in the literature with regard to joint line elevation. Mehta and colleagues state that "while joint line elevation may lead to increased midflexion laxity in patients with a flexion contracture, 2 mm of elevation may be well tolerated by patients, whereas 4 mm may be more problematic" [2]. In a computational study by Chalmers et al [17], it was found that with an additional 2 mm of distal femoral resection there was a significant increase in overall coronal laxity at flexion angles of 30°, 45°, 60°, and 90° [17]. Further, an increase in the distal femoral resection to 4 mm almost doubled the overall coronal laxity at 30°, 45°, and 60° but not at 90° [17]. Chalmers et al [17] note that the maximum laxity was seen between 11° and 33° of flexion, which is likely after the posterior capsular tension dissipates, and that the coronal laxity persists until approximately 70° of flexion. Thus, coronal laxity in midflexion is increased if the distal femur is additionally resected to gain extension during the primary TKA, and thus the advantage of elevating the joint line to gain extension should be carefully balanced against the disadvantage of increased coronal laxity in the midflexion range when performing a TKA [17].

With respect to prosthesis designs (Table 2), multiradii ones are designed to closely mimic the normal knee, whereas single-radius ones allow for uniform tension in the collateral ligaments throughout the range of motion [2]. Concerning midflexion instability and multiradii designs, the literature presents a conflicting picture: as reviewed by Rastogi and Marya [5], the original data comes from a kinematic study which showed mediolateral instability in the multiradii TKA in the midflexion range. Some later studies supported these results, yet others comparing single-radii and mutliradii TKA designs as well as long-term studies on multiradii knee designs did not show any concerns with regard to midflexion instability [5]. Further, two metaanalyses comparing outcomes with single-radius and multiradii designs showed no differences in outcomes between the two designs [18, 19]. Despite this, Mehta et al [2] comment that the radius of curvature is a potential source of midflexion instability, citing one study by Wang et al [20] in which increased laxity in the midrange of motion during transition from seating to standing was observed with multiradii designs. However, as concluded by the two aforementioned metaanalyses [18, 19], Mehta et al [2] also note the lack of consensus in the literature regarding implant design for midflexion stability. When asked for his opinion, Oussedik says: “It is certainly possible to engender midflexion instability with either single- or multiradii designs. Many of the studies do not account for the roll-back and pivoting motions seen in knee kinematics, which may also tension the surrounding soft-tissue envelope to aid stability. Careful intraoperative assessment whilst adhering to sound anatomical principles in implant placement are likely to be the safest ways in which to avoid trouble, whichever design is used.”

As regards medial collateral ligament laxity, this can be either preexisting or occur as a result of extensive soft-tissue release during primary TKA which causes iatrogenic injury to the medial collateral ligament [5]. The superficial medial collateral ligament is known to be isometric except for flexion between 30° and 50° when it lengthens and becomes lax [2]. The medial collateral ligament laxity is an important factor leading to midflexion instability, and it is recommended to perform an intraoperative assessment of varus and valgus stress testing at 30–60° of flexion, which is the midflexion range [5]. When medial collateral ligament laxity is the cause of midflexion instability, various treatment options are available (Table 2).