Foot and Ankle Anatomy and Biomechanics

Adam G. Miller, MD

Benjamin E. Stein, MD

Neither of the following authors nor any immediate family member has received anything of value from or has stock or stock options held in a commercial company or institution related directly or indirectly to the subject of this chapter: Dr. Miller and Dr. Stein.

ABSTRACT

This chapter examines the recent advances in anatomy and biomechanics of the foot and ankle. Achieving anatomic syndesmotic reduction has been a focus of recent literature. Appropriate radiographs are the first line of imaging a surgeon obtains. Ankle reconstruction and total ankle arthroplasty continue to be explored as an alternative to fusion. Gait analysis is a useful tool in research to identify patient outcomes.

Keywords: anatomy; biomechanics; gait; imaging; syndesmosis; total ankle

Introduction

An understanding of the structure of the foot and ankle is imperative to treat it successfully. The forces applied to these structures and how the foot and ankle move give us insight to normal function and treating pathology. This chapter examines the recent advances in anatomy and biomechanics of the foot and ankle.

Anatomy: Osseous

Twenty-eight bones make up the foot, creating a support for the ankle and rest of the lower extremity. One can think of the foot as a tripod where the calcaneus, first metatarsal head, and lesser metatarsal heads act as three pillars of support. Deviation from this creates uneven stress to the foot resulting in pathology. There is inherent stability and flexibility to the foot. With eversion, the chopart joints unlock and become parallel to allow for accommodation to the ground. When the foot inverts and prepares for impact (such as during running), the chopart joints are not parallel and stiffen the foot. The ankle has inherent bony stability due to the mortise and tenon joint design. The talus acts as the tenon fitting into the mortise comprised of the tibia and fibula.

The syndesmosis stabilizes the mortise while still allowing for rotation and translation of the fibula. Syndesmotic reduction and fixation techniques are still evolving. One concern is achieving anatomic reduction. A report of the bony anatomy of the incisura at the level of the syndesmosis may shed light to reasons for malreduction of syndesmotic injuries. In a series of postreduction ankle CT scans, malreduction was thought to be associated with native anatomy. A deep incisura was at risk for too much compression with a clamp resulting in overreduction (Figure 1). Anteverted and retroverted incisuras predisposed reductions to fail in those associated directions.¹ This information could give surgeons assistance in reduction techniques by identifying landmarks on CT.

Anatomy: Ligamentous

Ankle ligaments ensure the bony anatomy remain congruent through articulation. Lateral collateral ligamentous structures including the anterior talofibular and calcaneofibular ligaments are the most commonly repaired or reconstructed ligaments of the ankle.

Minimally invasive surgery relies on palpable and visual landmarks to assess accurate incisional placement. Lateral ankle instability surgery performed with a minimally invasive technique has gained popularity. The fibular obscure tubercle is a component of the distal fibula’s surface anatomy. It can be manually detected in 57% of patients and allows a surgeon identification of appropriate anterior talofibular ligament (ATFL) and calcaneofibular ligament (CFL) origins. The tubercle occurs “1.3 mm proximal to the articular tip of the fibula, 2.7 mm to the intersection of the ATFL and CFL, and 3.7 mm distal to the ATFL.”² Detailed understanding of surface anatomy may make minimally invasive surgery more accepted.

Figure 1 Axial depiction of tibiofibular bony relationship. (A) Normal congruent syndesmotic morphology. (B) Anteverted syndesmotic morphology. (C) Retroverted syndesmotic morphology.

Concurrent pathology with ankle instability is common, and therefore, ankle arthroscopy can be used to identify and treat osteochondral lesions and loose bodies. In addition, arthroscopy may be used to identify instability. Arthroscopic visualization of the ankle is utilized during minimally invasive Brostrom techniques. A feasibility study confirmed that one can see the ATFL and CFL safely through arthroscopy of the ankle.³

While bony anatomy in the midfoot is responsible for adding the stability of the midfoot, the Lisfranc ligament is crucial to maintaining articular relationships and ultimately the contour of the arch in the coronal and sagittal planes. Injury to the Lisfranc ligament—if determined to be unstable—requires surgical intervention. Although open reduction and internal fixation and fusion are both acceptable options established in literature, no clear treatment guidelines exist in choosing which option is best for which patient.

Anatomy: Tendinous

With bony tripod of the foot stabilized by ligamentous structures, tendons move and accommodate this tripod to surfaces. Pathology in a tendon not only creates pain and possibly weakness but also imbalance to the foot. The peroneal brevis tendon is a primary evertor of the foot balanced by the posterior tibial tendon’s inversion pull. Injury may disrupt this. A relatively new fixation technique uses fibular fixation with a retrograde intramedullary nail. This puts the brevis tendon at risk coursing in the retrofibular groove. With insertion of the nail, the peroneus brevis is most at risk for injury. In an anatomical study, on average, the brevis tendon was 5 mm from the nail path. The superficial peroneal nerve was also at risk variably with anterior to posterior screws placed through the nail.⁴

Tendon imbalance can also be in the sagittal plane with dorsiflexion and plantarflexion of the ankle. The Achilles tendon originating with the soleus and gastrocnemius muscle bellies provides a strength and pull that may result in equinus position of the ankle. Gastrocnemius release is common, and several methods exist. A proximal medial head release depending on location of incision can encroach upon several structures. A proximal medial gastrocnemius release brought semimembranosus tendon, popliteus artery, and tibial nerve all within 11 mm of the incision in 100% of specimens in one study. A more distal alternative medial
gastrocnemius release incision encountered only saphenous vein and nerve with a large variability in distances (5 to 18.6 mm).⁵

Tendon transfers may help to balance a foot and ankle. The flexor halluces longus (FHL) tendon is often transferred for Achilles support and sometimes to the lateral foot in patients with deficient peroneal tendons. The flexor digitorum longus (FDL) is commonly used to replace or augment the posterior tibial tendon when reconstructions are performed. Plantar tendon harvest often encounters chiasma plantare between the flexor halluces longus and flexor digitorum longus. In a series of 50 cadaveric dissections, 61% of FHL tendons provided one or more attachments to the FDL (second and third being most common). Thirty nine percent of specimens had direct intertendon connections. Overall, the FHL is involved in 97% of second tendons and 53% of third tendons.⁶

Anatomy: Neurovascular

With any intervention, intimate knowledge of nerve location is essential. The superficial peroneal nerve has variability in its exit location from lower leg fascia proximally to distally requiring caution during dissections. This nerve is protected during creation of the anterolateral arthroscopy portal and can be encountered in lateral ankle approaches. Recently, location of the nerve in relation to the ankle was reviewed. The mean distance from the fibular head to the emergence of the SPN is 24.6 cm in a study of 10 cadaveric specimens (Figure 2). The mean distance from the lateral malleolus to the nerve at the level of the ankle was 4.68 cm.⁷

Figure 2 Illustrations showing the superficial peroneal nerve average location.

Another anatomic study evaluated SPN location in relation to the distal fibula. When performing minimally invasive techniques one can palpate the distal fibula and determine relative safe zones for dissection. Lateral ankle instability performed minimally invasive should not dissect 22 mm past the distal fibula anteriorly in an effort to protect against superficial peroneal nerve injury.⁸

The posterior tibial nerve lies underneath the laciniate ligament in the tarsal tunnel and arborizes into the calcaneal, medial plantar, and lateral plantar branches. The cause of tarsal tunnel syndrome is incompletely understood. In 2016, an anatomical study evaluated the neurovascular relationships in the tunnel. Their conclusion was that an extensive vascular supply to the posterior tibial nerve in the tarsal tunnel may cause vascular congestion and leave the nerve susceptible to symptomatic compression more than other nerve compression relationships.⁹

Imaging

As in many of the orthopaedic specialties, diagnostic imaging is an essential component of the medical workup for much of the pathology encountered by the
foot and ankle specialist. In an era of multiple imaging modalities, it is incumbent on the treating physician to understand the intricacies of these various modalities and to utilize them when appropriately indicated.

Plain Radiographs

Plain radiographs of the foot and ankle serve a critical role in the accurate assessment and diagnostics of foot and ankle pathology. In most clinical scenarios, plain radiographs are the initial study of choice in imaging workup. These studies are low-cost and easily obtainable in the office setting and provide important diagnostic data with regard to assessing alignment, trauma, degenerative, and potential neoplastic conditions. Weight-bearing radiographs are particularly useful and are strongly preferred with the exception of postoperative or traumatic situations that may warrant non-weight-bearing radiographs.

A standard series of weight-bearing radiographs of the foot includes AP, lateral, and oblique radiographs (Figure 3). Depending on the clinical context, additional “special views” may also be indicated. These may include dedicated radiographs of the toes, sesamoids (Figure 4), and possible stress radiographs of the foot when indicated (ie, suspected unstable Lisfranc injury).

A standard series of weight-bearing radiographs of the hindfoot and ankle includes AP, lateral, and mortise radiographs (Figure 5). As with the foot, there are additional views that may be obtained depending on the context. Stress radiographs of the ankle are performed in determining stability of certain ankle fracture patterns as well as in assessing competency of ligamentous structures in more chronic scenarios (Figure 6). An axial or Harris view of the calcaneus can be obtained when indicated to better visualize the bony anatomy of the calcaneus. The Broden view of the hindfoot is a special view used in assessing the posterior facet of the subtalar joint. This can be particularly useful when assessing an intra-articular calcaneal fracture or a subtalar coalition (Figure 7). The hindfoot alignment view or Saltzman view is now also commonly obtained when evaluating the axial alignment of the hindfoot in relation to the ankle above. This is particularly important for preoperative planning purposes in corrective hindfoot deformity surgery (Figure 8).

MRI

MRI of the foot and ankle can provide useful data in evaluating acute or chronic trauma, impingement, osteochondral injuries, arthropathies, occult fractures or stress reactions, osteonecrosis, soft-tissue and osseous tumors, infection, nerve entrapment syndromes, and tendon pathology. When indicated, it can be a critical part of determining the diagnosis and guiding appropriate treatment. There are a variety of MRI systems available, but most commonly, midfield (1.5-T)
and high-field (3-T) systems are used in the evaluation of the foot and ankle.¹⁰ The higher field systems allow for higher resolution and are thus preferable in most circumstances for the foot and ankle specialist. There are specific scenarios such as when previous internal fixation is present that a midfield (1.5-T) MRI system would be preferable.