Anatomy–mechanism of injury–biomechanics

In recent literature, there is a growing interest in the biomechanical behavior, function, injury mechanism, and individualized treatment of high ankle sprains. Thoughtful interpretation of these publications on the distal tibiofibular syndesmosis may raise questions about striking similarities with the anterior cruciate ligament (ACL) function and mechanism of injury to the knee. Consequently, the question then is whether we are looking at identical entities, a continuum or separate injuries? [6, 9, 10, 17, 18, 26, 28].

A rapid pivoting ankle dorsiflexion in valgus (pronation) with a forceful external rotation of the calcaneo–talar–fibular complex relative to the tibia on a fixed forefoot is the most common mechanism of a high ankle sprain. Consequently, this may lead up to a distal ankle syndesmotic injury. As the talus rotates in the mortise, the fibula rotates externally and moves posteriorly and laterally. This mechanism separates the distal tibial and fibula with sequential tearing of the anterior inferior tibiofibular ligament (AITFL), the interosseus ligament (IOL), and in conjunction the (deep) deltoid ligament. In gross instability, the posterior inferior tibiofibular ligament (PITFL) might be involved, presenting as a ligament tear, tibial avulsion, or edema [4, 5, 18, 29, 30]. Striking similarities of this ankle mechanism with the classic non-contact valgus rotation trauma of the femur (external rotation) relative to the tibia in the knee, causing an ACL rupture in the knee, were observed by us [11, 13, 21]. In both mechanisms, a pivoting moment on a fixed forefoot or fully fixed foot is seen in respectively distal syndesmotic ankle or ACL knee injuries. Can it be hypothesized that this is a crucial factor to sustain one injury over another? (Fig. 1a, b).

Fig. 1
figure 1

a Mechanism of ankle syndesmosis injury. With the forefoot fixed in the grass and valgus/pronation of the hindfoot, the deltoid ligament ruptures. External rotation of the (for/mid) foot/talus/fibula complex causes rupture of the AITFL−IOL−(PITFL) (green + green arrow). Internal rotation of body and tibia (blue + blue arrow). Axial body load (red arrow). b Mechanism of ACL injury. Whole foot fixed in the grass. Valgus moment. External rotation of the femur and internal rotation of the tibia causing the postero-lateral bone bruise on the tibia (subluxation) (green and blue arrow). Axial load of body weight (red arrow)

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Latest biomechanical insights in syndesmotic injuries with sequential cutting of the AITFL, IOL, and in conjunction the deltoid ligament revealed a multidirectional instability occurring in the coronal, sagittal, and transverse planes. These findings suggest an antero-posterior translational and rotational instability of the distal fibula relative to the tibia [18]. In our opinion, one should not reason too far to appreciate quite striking similarities with the ACL-deficient knee. Can it be assumed in this comparison that the AITFL/IOL complex equals the ACL and the deltoid ligament (as a rotatory constraint) equals the eccentric anterolateral ligament (ALL)/Kaplan fibers in the ACL story? [8, 15, 18, 21].

Modern sports rehabilitation focusses on a holistic kinetic chain approach, taking into account neighboring joint and muscle groups. Is the causal factor to differentiate between a syndesmosis ankle and ACL knee injury, the position of the foot, the surface texture of the pitch, or is it the weight-bearing status of the injured limb or movement of the upper body? [1, 11, 13, 21] Additionally, assumptions are made in the recent literature that a limited hip range of motion may alter ACL injury risk; looking down the chain of kinetics, one may wonder if there is a relation between the above-mentioned syndesmotic ankle and ACL knee injuries [2, 3, 25].

Diagnostics

The anterior drawer, and Lachman and pivot-shift tests are key clinical indicators in the diagnosis of an ACL knee rupture. These three clinical tests are helpful to evaluate antero-posterior and rotational instability in the ACL-deficient knee [14]. Similar to these tests, clinical examination of the ankle syndesmosis focusses on both types of instability by means of, respectively, the fibular translation test and the external rotation test, with the latter being the most sensitive, with the lowest false-positive test results [24, 27]. To our knowledge, no combined test is yet available to appreciate combined A-P and rotational ankle instability.

Magnetic resonance imaging (MRI) is the current modality of choice to visualize both the injured knee ACL as ankle syndesmosis ligament injuries. In acute ACL injuries, the femoral and postero-lateral tibial bone bruises are the pathognomonic signs of a sustained pivot-shift injury [7, 20, 23, 31]. Recent work of Randell et al. [22] revealed similar bone edema patterns in the posterior tibial plafond and talus, contributing to the suggested pivot-shift ankle mechanism concept. Quod erat demonstrandum?

Treatment and outcome

Recurrent ankle instability is known to ensue and eventually lead to premature ankle arthritis. In line, chronic ACL-deficient knees may lead to secondary cartilage/meniscal damage and early onset osteoarthritis. Therefore, a timely diagnosis of unstable syndesmosis ankle and ACL knee injuries is essential. Until today, no predictive clinical knee or ankle tools are available to differentiate ‘copers’ from ‘non copers’ [12]. Bearing these consequences of chronic instability in mind, ACL knee reconstruction is the current treatment of choice in the active patient with reproducible and good-to-excellent results. With regard to surgical techniques, the past has proven that ACL repair failed on the mid and long term, with an ongoing search for suitable techniques. Treatment of choice remains reconstruction with lateral extra-articular tenodesis in case of significant rotational instability and pivoting sports [19]. Current techniques for treatment of ligamentous syndesmotic injuries are based on augmentation techniques to restore joint congruence and stability. Autologous ankle reconstruction techniques of the AITFL and/or IOL remain scarce without any evidence of superiority or added value [16].

Reflections

Reading the emerging literature on the high ankle sprain presents us with new insights but raises additional questions at the same time. When looking at the anatomy, biomechanics, diagnosis, and treatment of both and syndesmosis ankle injuries, the authors came across striking similarities. Whether identical entities, a continuum or separate injuries are faced; the aforementioned questions present a need for future biomechanical research on the potential link in pivot-shift injury of knee and ankle.