Evaluation and treatment of extension and recurvatum instability

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Instability after total knee arthroplasty (TKA) was introduced in Part 1  of this series. Two different and relatively uncommon forms of instability are extension instability and genu recurvatum. Hyperextension and recurvatum describe the same type of deformity, though for practical terms extension instability refers mostly to the result of the surgical technique used during the primary TKA (sometimes also coined as ‘instability due to bone resection’), whereas genu recurvatum describes the preexisting hyperextension. This preexisting hyperextension is usually associated with neuromuscular disease, such as poliomyelitis, or can occur in patients with a valgus deformity and a tight Iliotibial band, or in cases of cruciate and collateral ligament laxity, such as in rheumatoid arthritis [1]. A thorough understanding of the etiology of the instability is crucial to determine the correct course of treatment and the choice of implant used during revision TKA. In this article, Beatriz Montoya-Ortiz from the Clinical Care Center for Joint Replacement at Clínica El Rosario in Medellín, Colombia shares her clinical experience with us and takes us through the differences between extension instability and genu recurvatum, how to evaluate the patient, and considerations for treatment.

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Beatriz Montoya-Ortiz

Clínica El Rosario
Medellín, Colombia

Extension instability

Extension instability can be described in patients who have a knee which is well balanced in flexion but loose in extension, usually with an elevation of the joint line [2]. It can be subclassified into the two types symmetric or asymmetric instability, with the latter being more common. Extension instability can be characterized by the shape of the extension gap, with a rectangular gap associated with symmetric, and a trapezoidal gap with asymmetric extension instability [3]. As Beatriz Montoya notes, the space between the transverse cut of the distal femur and the transverse proximal tibial cut (the extension gap) should be equivalent to the space between the posterior femur and the transverse proximal cut (the flexion gap) for the ligaments to be balanced (Figure 1). However, when preparing for the bone resection, if the alignment of the cutting guide changes or if an underlying angular deformity is not corrected, it changes the plane of resection, thus creating a mismatch between the gaps leading to instability.

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Figure 1. A sagittal view of the knee in flexion and extension showing equal flexion and extension gaps.

Symmetric instability

In the case of symmetric extension instability, there has usually been excessive distal femoral or proximal tibial resection, and thus the extension gap cannot be appropriately filled by the components [4]. Excessive proximal tibial resection affects both extension and flexion gaps and can be managed typically using a thicker polyethylene insert that will fill the space equally in flexion and extension [3]. However, this is not appropriate when there is excessive distal femoral resection, because in this case the extension gap is primarily affected leaving the flexion gap unchanged, and a thicker tibial insert would raise the joint line and cause flexion gap tightness limiting deep flexion [1, 3]. As discussed by Al-Jabri et al [2], this can lead to different problems such as pseudopatellar baja or impaired patellofemoral biomechanics and loss of isometry in the knee ligaments, which could contribute to a midflexion instability [2]. It is thus recommended to correct the joint line height, balance the extension gap, and correct patellofemoral understuffing with distal femoral augments added to the prosthesis [3, 5]. When approaching a patient with symmetric instability following a TKA, Montoya notes that after bone resection, osteophyte removal, and soft-tissue release of the contracted structures, gap balance is tested imparting full range of motion and mediolateral stress at both 0° and 90°. If a symmetrical laxity is noticed both in flexion and extension, then a thicker polyethylene insert will be used increasingly until balance is achieved. However, if the stability testing shows that the knee is balanced in flexion but very loose in extension, then distalization of the femur is ensued with the use of distal femoral augments to restore the joint line, thus decreasing primarily the extension gap.

 

Asymmetric instability

Asymmetric instability occurs more frequently than symmetric instability and has a different root cause, being the result of an incomplete correction of an angular deformity in the coronal plane or damage to the collateral ligaments during surgery [4]; the result is that the patient may have asymmetric ligamentous balance. In comparison with symmetric instability, where the extension gap is rectangular, the extension gap in asymmetric instability is trapezoidal (Figure 2) [6].

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Figure 2. An illustration showing a trapezoidal extension gap. The trapezoidal extension gap can be seen on the anteroposterior radiograph of the knee in extension. LCL, lateral collateral ligament; MCL, medial collateral ligament.

When a varus or valgus deformity needs to be corrected during the procedure, several strategies can be used to prevent asymmetric instability. For those who propose mechanical alignment, the method is to release the contracted soft-tissue structures in order to restore alignment coronally. In the case of the varus knee, release of the tight medial structures can be completed in different ways, but the best option, as described by Insall et al [7], is subperiosteal elevation of the superficial medial collateral ligament (MCL) from the tibia leaving the pes anserinus tendons intact. In the case of the valgus knee, the lateral structures are taut and the medial structures are loose. Here, the recommended technique is progressive multiple stab incisions of tight lateral structures—the so-called ‘pie crusting’ technique [8]—during which the knee is fully extended and a lamina spreader is placed in the extension space as several stab incisions are made with a surgical blade along the lateral side of the knee, starting with the structures that are palpated as being the tightest and intermittingly testing to prevent overrelease.

For those who propose kinematic alignment, the concept is to restore the patient's native anatomy and joint line. The aim is to replace the removed bone or cartilage with an implant of the exact same thickness. This does not create gap imbalances and therefore ligament releases are not required [9]. Kinematic alignment is performed with patient-specific guides or with robotic assistance, and balancing is achieved through osteophyte removal and bone resection, according to the angular deformity present. For a typical varus knee reconstructed with 8 mm thick distal and posterior femoral condyles on the femoral component, the thickness of the distal medial resection should be 5 mm, the posterior medial resection should be 6 mm, and the distal and posterolateral resections should be 7 mm. If the femoral component is 10 mm thick, then each bone resection should be 2 mm thicker than the cuts for an 8 mm thick femoral component. For a typical valgus knee, the pattern of the bone resections should be reversed with the thinner resections lateral and the thicker resections medial. If a femorotibial soft-tissue imbalance persists (tightness and/or excessive laxity) even though the integrity of the knee soft-tissue envelope is still respected (no MCL or popliteal section), this is often because the tibial cut is improper. The solution is therefore to perform bone recuts by using specific recut guides that easily enable additional degrees of varus/valgus/slope to be made or an additional 2 mm of tibia to be cut [10].

However, if any of the chosen methods for balancing does not stabilize the knee, then a varus or valgus constrained condylar knee implant may be needed [5, 6]. Miralles-Muñoz et al [6] conducted a retrospective study comparing rotating hinge knee and constrained condylar knee implants for revision TKA in patients with symptomatic extension instability after primary TKA with a cruciate-retaining implant [6]. Here, patients were diagnosed with TKA instability according to their symptoms, clinical examination, and x-rays. Extension instability was categorized according to the Knee Society Score, whereby patients had a mediolateral gap during varus-valgus stress maneuvers when the knee was in extension. The study showed that both types of implants led to good outcomes for the treatment of symptomatic extension instability [6]. Although there were no statistically significant differences, the authors noted that there were fewer complications with the constrained condylar knee implant than with the rotating hinged one, which appeared to be relevant [6].

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Tips and tricks

We asked Montoya for her tips for preventing instability in extension:

  • Pay attention to staged soft-tissue release and proper balancing of ligaments before moving on to greater bone resection; the most common cause of asymmetric extension instability is insufficient correction of deformity.
  • Avoid elevating the joint line by minimizing distal femoral bone resection.
  • Consider the tibial slope of your implant and inclination of your tibial slope.
  • Consider the very strict plane of resection of the tibial cut if you are angling the blade and not using robotics.
  • Adopt a ‘less is more’ approach when performing the resection. The tibial guides allow you to cut more or less bone, depending on the preoperative planning and the tightness of the gaps. Take your time—you can always cut more, if necessary, but you cannot go back if you resect too much.

Hyperextension after total knee arthroplasty can impact
quality of life

Although TKA has been shown to positively impact patient outcomes, such as functional performance [11], when hyperextension occurs after TKA, this can have a negative impact on the outcomes for patients [11]. In one center, hyperextension after TKA was reported in as many as 15.8% of patients at 2 years [11]. Siddiqui et al [11] aimed to understand what level of hyperextension was functionally acceptable. They found that up to 5° of hyperextension was still functionally acceptable, whereas when hyperextension exceeded 5° this significantly impacted quality of life, particularly physical function and general as well as mental health [11]. Furthermore, this level of hyperextension could eventually necessitate revision surgery [11].

Hyperextension after TKA can be classified based on the proposed grading system by Siddiqui et al (Table 1) [11].

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Table 1. Grading system for knee hyperextension according to Siddiqui et al [11]. SF-36, short-form health survey with 36 questions.

In contrast to extension instability, genu recurvatum is a clinical feature observed in patients with neuromuscular disorders [12], though it can also be caused by equinus deformity of the ankle, a reversed tibial slope seen in patients with old tibial plateau fractures or previous high tibial osteotomy, severe genu valgum deformity, ligamentous deficiency, and rheumatoid arthritis [12]. In patients with rheumatoid arthritis, genu recurvatum occurs with cruciate and collateral ligamentous laxity [13]. When a high tibial osteotomy is the cause, impaction of the anterior tibial cortical bone creates an anterior slope of the proximal tibial plateau [13]. In some patients with genu recurvatum, a fixed valgus deformity is the result of iliotibial band contracture [13].

The most common neuromuscular disorder causing genu recurvatum is poliomyelitis [3]. In patients with poliomyelitis, there is a profound quadriceps muscle weakness, which leads the affected knee to develop a valgus deformity, collateral ligament and posterior capsular laxity, external tibial rotation, and recurvatum deformity [3, 14]. As Montoya explains, the quadriceps muscle is needed to enable a person to stand up straight; patients with a weak quadriceps muscle have an inherent need to stabilize themselves by locking the knee in hyperextension or sometimes even pushing the thigh backwards, using the back and gluteal muscles for balance, thus preventing the patient from falling forward. It is especially challenging because this hyperextension should not be corrected completely when performing the TKA, since it is required to both compensate for the inefficient quadriceps power and to be able to walk.

 

Treatment choices in genu recurvatum

Genu recurvatum is difficult to treat and there are various methods of treatment discussed in the literature, although very few studies have reported outcomes after TKA in genu recurvatum and existing literature is limited by both the low incidence and short follow-up. Cottino et al [3] propose three methods for correcting genu recurvatum. The first approach allows the use of a standard primary implant, by first tightening the extension gap by underresecting the distal femur, using a thicker polyethylene insert and placing the femoral component in slight flexion [3]. A study by Seo et al [15] reported satisfactory short-term results obtained in patients with genu recurvatum (≥ 5°) undergoing primary TKA when a smaller distal femoral resection was made and a thicker insert used. However, in this study none of the recurvatum cases were related to neuromuscular disease. Mesnard et al [16] have also assessed the long-term outcomes of using a posterior stabilized tricondylar implant in patients with genu recurvatum of ≥ 10°. At 5 years after TKA, good clinical and radiological outcomes were found with no differences between the patient groups in terms of radiological outcomes, revision rates, or complications [16]. Notably, in patients with preoperative genu recurvatum, no instability was observed; however, as in the previously cited publications, genu recurvatum associated with neuromuscular disease was excluded from the study [16].

The second approach to genu recurvatum proposes to tighten the collateral ligaments in extension, which achieves a tighter extension gap and thus prevents a hyperextension deformity [3]. Cottino et al [3], however, note that the quality of the soft tissue in these patients may be poor and there is a chance that further stretching and possible recurvatum recurrence may occur. The third and preferred approach from Cottino et al [3] is to use a rotating hinge TKA with an extension stop to reduce the risk of postoperative hyperextension instability. However, Petrie et al [5] commented that persistent hyperextension may lead to implant failure when a rotating hinge TKA is used. Petrie also noted that long-stemmed posterior stabilized or varus/valgus constrained implants have shown moderate success [5].

When asked what she thinks about using rotating hinged knee implants in this setting, Montoya explains, in patients with genu recurvatum associated with neuromuscular disorders the current recommendation is to go straight to a hinged implant. This is because it provides immediate instability as it does not allow for any kind of mediolateral laxity and so it restricts movement to only flexion and extension in the sagittal plane, but it provides an inbuilt anterior stop that does not allow the knee to go into excessive hyperextension. The concern always remains that the high level of constriction it provides transfers the shearing forces from the joint to the bone-cement interface, accounting for its higher failure rate reported in the literature. However, it has been shown in patients with poliomyelitis that both knee function and quality of life improve with TKA, despite there being a high rate of revision [17, 18]. So if we consider that many patients with neurological conditions will have lower activity levels and lower expectations than the average patient undergoing TKA and that modern rotating hinge implant designs promise improved survivorship, providing a pain-free stable knee often takes precedence over fears of long-term aseptic loosening in this particular group of patients [14]. Rotating hinge knee implants are recommended by Digennaro et al [17] also for use in patients with quadriceps muscle weakness, such as in poliomyelitis, as this may be a good alternative to maintain physiological knee kinematics. It should be noted that modern rotating hinge knee implants closely mimic physiological kinematics which, as a result, reduces stress at the implant-bone interface [18]. The use of these implants could reduce the revision rate due to knee instability [17]. The use of a rotating hinge knee implant has also been shown to be successful even in a case of severe recurvatum to over 55° [19]. A metaanalysis by Prasad et al [18] also concluded that the use of constrained implants, such as posterior stabilized or rotating hinge knee implants, is recommended where quadriceps strength is poor (ie, less than antigravity), although it is important to ensure good intraoperative soft-tissue balancing and restoration of alignment in order to achieve a good outcome. Pomeroy et al [14] also note that hyperextension can be overcome when using a constrained condylar knee implant by underresection of the distal femur or increased posterior tibial slope, commenting that this reduces an imbalance in the flexion and extension gaps [14].

 

De novo genu recurvatum after total knee arthroplasty

The appearance of de novo genu recurvatum after TKA is a very difficult scenario. The literature here is limited; Mortazavi et al [20] report on four patients who experienced late onset de novo genu recurvatum after TKA (10–20°). These patients did not have an underlying neuromuscular disorder or poliomyelitis, only one had rheumatoid arthritis and all experienced generalized ligamentous laxity [20]. Mortazavi [20] took an alternative approach to treatment in that patients were treated with an isolated polyethylene exchange with good outcomes. He concluded that when patients have ligamentous hyperlaxity, using a thicker polyethylene liner at the time of primary TKA may be a suitable option for preventing late onset genu recurvatum in these patients [20].

 

Approaching the clinical examination in patients with suspected instability

“Instability is a symptom, not a diagnosis, and the diagnosis of the underlying cause is not as straightforward as one thinks,” says Montoya. “The clinical manifestations of instability can be very subtle; the patient may present with a knee that is a little swollen and there may be some discomfort.” As Montoya explains, “these symptoms can also be the result of many things, such as a lack of rehabilitation or even an infection; this means, that by the time there is a diagnosis, the knee is already unstable. Therefore, it is important to undertake a thorough history to try and understand the patient’s complaint, a complete clinical evaluation, and radiographic analysis”.

The general clinical examination for instability should include an assessment of any extraarticular causes of knee instability such as neuromuscular disorders, hip or ankle deformities, or areas of tenderness [3, 21]. Montoya comments that knee alignment will be apparent, but the spine and hips must be observed for problems outside the knee. Extension instability may manifest as varus-valgus, recurvatum, or both. Hyperextension results from component subsidence with bone loss, technical mismatch of flexion and extension gaps (an oversized femoral component leading to a tight flexion gap and the selection of a thin polyethylene insert), or the quadriceps weakness described above that leads to a recurvatum gait. It is important to watch the patient walk and observe for the presence of a ‘thrust’ or a paradoxical movement of the knee in motion. Varus-valgus testing should take place in full extension, 30° of flexion, and 90° of flexion [3, 21]. The surgeon must focus specifically on the sensation of a valgus stress, calculating primarily if there is an intact collateral ligament, and conversely with a varus stress if there is a sense of intact lateral structures. Anteroposterior drawer maneuvers (with the patient supine) are different after arthroplasty surgery: a posterior stabilized prosthesis should exhibit a hard mechanical end point to the posterior drawer as the spine and cam mechanism engages. Absence of this mechanical stability suggests that the tibial polyethylene has broken off—a typical complication of recurvatum deformity as the anterior spine strikes the edge of the femoral trochlear groove. The patient should be evaluated for infection if pain is present using the following diagnostic tests: erythrocyte sedimentation rate and C-reactive protein [3, 21].

The radiographic analysis should include:

  • Anteroposterior varus-valgus stress x-rays to assess collateral ligament status and check if deformities can be reduced [3, 21]
  • Lateral x-rays in full extension, 90° of flexion, and full flexion to measure tibial translation on the femur, implant positioning, flexion gap and tibial slope [3, 21]
  • Full-length weight bearing x-rays to assess component positioning [3, 21]
  • Patellar views [3, 21]

A computer tomography scan is indicated if component malrotation is suspected [3, 21]. Magnetic resonance images are rarely indicated and can be difficult to obtain, although they may be used for soft-tissue evaluation and component rotation [3, 21].

 

Conclusion

Extension instability and genu recurvatum are rare forms of instability and are difficult to treat. It is important to differentiate between extension instability, which occurs after TKA, primarily because of surgical technique, and genu recurvatum, which occurs in patients with quadriceps weakness as a result of neuromuscular disorders. Prevention of extension instability is crucial, and care must be taken during the primary TKA. In patients with genu recurvatum and quadriceps weakness, a constrained implant such as a rotating hinge knee implant has shown good clinical outcomes.

Hyperextension is also a risk factor for revision TKA. By using the grading system shown in Table 1, Siddiqui et al were able to conclude that if a patient had hyperextension at 6 months, they were 6.5 times more likely to have recurvatum at 2 years compared to those patients who had no hyperextension [11]. This means that postoperative hyperextension is unlikely to correct over time; nearly half of the patients with grade 2 hyperextension at 6 months progressed to grade 3 at 2 years [11].

Siddiqui et al also noted that global laxity, which may present as hyperextension of > 5°, can lead to revision TKA [11]. As with extension instability, global laxity can occur through inadequate gap balancing or inadequate varus or valgus alignment, undersizing of the polyethylene insert, or an excessive distal femoral resection or posterior release [11]. Thus, to achieve good functional outcomes, it is important for the surgeon to correct any hyperextension by ensuring that there is good intraoperative alignment and gap balancing [11].

 

Genu recurvatum

Genu recurvatum is a term which is used to describe hyperextension of the knee where knee extension is > 5° [12]. Genu recurvatum (Figure 3) is very rare occurring in only 0.5–1% of patients undergoing a TKA [2, 3], though more recent studies utilizing navigation technology have shown that genu recurvatum can be present in as many as 3.5–11.8% of patients [12].

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Figure 3. X-rays showing genu recurvatum.
Read more
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Part 1 | Etiology and classification of the unstable total knee arthroplasty

Read more
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Part 3 | Evaluation and treatment of flexion midflexion and global instability

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Additional AO resources

Access videos, tools, and other assets.

Contributing experts

This series of articles was created with the support of the following specialists (in alphabetical order):
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Dario E Garin

Hospital Ángeles
Tijuana, Mexico

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Beatriz Montoya-Ortiz

Clínica El Rosario
Medellín, Colombia

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Sam Oussedik

AO Recon Joint Preservation Knee Curriculum Taskforce
University College Hospital London
London, UK

This issue was written by Lyndsey Kostadinov, AO Innovation Translation Center, Clinical Science, Switzerland.

References

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  17. Digennaro V, Manzetti M, Bulzacki Bogucki BD, et al. Total knee replacements using rotating hinge implants in polio patients: clinical and functional outcomes. Musculoskeletal Surg. 2022.
  18. Prasad A, Donovan R, Ramachandran M. et al. Outcome of total knee arthroplasty in patients with poliomyelitis: A systematic review. EFORT Open Rev. 2018;3(6):358–362.
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