The human ankle is an exquisitely designed machine, and among its intricacies lies a small but significant feature known as the incisura fibularis. You may never have heard of it, and to be fair there’s not much to discuss over the coffee table, but to the clinician or surgeon, the incisura fibularis is a tremendously important and profound aspect of the ankle joint. This article will look at the structure and function of the incisura fibularis, how it is affected during normal daily activities and why it is such a vital component to its contact surface in the event of ankle joint pathology.
Overview of the Incisura Fibularis Structure
The incisura fibularis is a concavity that makes up the distal end of the tibia (the larger of the two bones comprising the lower leg). It forms the socket into which the lower end of the fibula rests – the distal tibiofibular joint. Were it not for this notch, the two bones would not stay in close contact at the ankle.
Interestingly, the joint between these two bones is not a typical joint, but a fibrous syndesmosis, held in place by a number of strong ligaments (discussed later).
Etymology and Terminology
The origin of this term is Latin. Incisura=notch and fibularis=fibula. Loosely translated this is the fibular notch; this is the term that you are more likely to hear on a clinical basis.
Older anatomical descriptions often use the term ‘peroneal notch’, which is from the Greek word for fibula. All three names refer to the same structure.
Basic Anatomical Description
Located at the lateral surface of the distal tibia is the notch, a shallow concavity accommodating the inferior fibula. This is defined by anterior and posterior borders of elevated bone margin, which, in places, form the anterior and posterior incisural lips of the tibia. Depth of this groove can be very variable, with associated effects upon joint stability.
The edge of the articular surface is accompanied by a rough fibrocartilaginous lining.
Detailed Anatomy and Variations
Anatomical knowledge of the incisura fibularis is far more detailed than its location alone. Its precise shape (depth, width, and direction) directly impacts how stable the fibula is when subjected to loads during weight-bearing androtation. These aspects have been examined in detail to understand their role in injury mechanisms and clinical procedures.
Shape, Orientation, and Bony Landmarks
Most depictions of the incisure fibularis describe the shape to be approximately triangular in transverse section with the apex directed anteriorly and the base directed posteriorly. The anterior tubercle(Chaput’s tubercle) and the posterior tubercle(Volkmann’s tubercle) are the two prominences identified on the external aspect of this notch. These prominences are points of bony attachment of the anterior and posterior tibiofibular ligaments which are thought to be the main stabilizing structures of the syndesmosis.
It is also observed that the direction of the trochlea is obliquely directed from anterolateral to posteromedial.
Anatomical Variants and Their Classification
There is quite a bit of range in how deep or shallow the notch actually is from individual to individual. Some classification schemes identify three main types: a shallow flat notch; a moderate groove; and a deeply concave type. A shallow notch may diminish the mechanical restraint of the fibula and result in an increased likelihood of syndesmotic instability following ligament disruption.
In a very deep notch, the opposite may be true, and the technique of reduction may be complicated. These classifications are best performed with CT scans rather than plain X-ray.
Biomechanical Role and Function in the Ankle
The incisura fibularis is not a simple receiving station. It contributes directly to the function of the ankle during gait, running and loading. The shape of the notch of the tibia in conjunction with the ligaments of the syndesmosis influence the way in which the joint responds to forces being transmitted through it and the extent of axial rotation of the fibula on the tibia.
Articulation with the Fibula and Syndesmosis
At the distal tibiofibular joint, the fibula doesn’t merely sit in the notch—the fibula glides. In dorsiflexion of the ankle, the fibula externally rotates and moves proximally and laterally by a few millimeters. The incusura fibularis permits this motion while the syndesmotic ligaments—the anterior inferior tibiofibular ligament, posterior inferior tibiofibular ligament and interosseous ligament—manage the extent of this motion.
Normal ankle kinematics require all of these components to work in unison. Disrupt any element and there is a problem.
Influence on Ankle Stability and Load Transmission
The notch itself of course also adds to the rotational stability to some extent. The deeper the more well defined the incisura fibularis is the more bony constraint is provided so the ligaments are not required to prevent more fibula movement. The transmission of loading across the ankle joint is also pertinent: it has been shown that during weight bearing loading through the ankle the fibula transmits about 17% of the axial load and so the geometry of the notch can affect the efficiency of the load transmission.
Altered kinematic behavior may result in an increased rate of cartilage degeneration.
Clinical Significance and Common Pathologies
From a clinical perspective the incisura fibularis is pivotal in ankle trauma. Fractures around the ankle frequently disrupt the syndesmosis. The position of the fibula in the notch following reduction has implications for future prognosis.
Incisura Fibularis in Ankle Fractures
High ankle sprains (technically syndesmotic injuries) are injuries to the syndesmotic ligaments that hold the fibula to the tibia at this joint. When severe it can lead to lateral dislocation of the fibula out of the notch, wider mortise and instability of the entire ankle. Bimalleolar fractures often include the posterior tibial lip (Volkmann’s fragment) which forms part of the border of the notch.
An inadequate reduction of the fibula within the incisura fibularis following surgery is a recognized cause of poor function and post traumatic osteoarthrosis.
Imaging and Assessment in Clinical Practice
Plain radiographs can provide evidence of syndesmotic injury – classical signs include widening of the tibiofibular clear space or medial clear space. However CT scan is the best modality for visualization of 3D notch shape and the position of the fibula. MRI is useful in assessment of ligamentous integrity, avoiding exposure to X-ray radiation.
Surgeons now use comparison of CT images with the uninjured side to verify an accurate reduction, as the shape of the notch can vary significantly between patients.
Surgical and Practical Considerations for Clinicians
Surgical management of syndesmosis injuries involves accurate reduction of the fibula within the incisura fibularis. Even a slight displacement can alter ankle biomechanics enough to produce pain and dysfunction. Clinicians practicing in the fields of orthopedics or sports medicine should be familiar with not only the anatomy of the notch but also its significance as a guide in intraoperative management.
Anterior Incisura Corner as a Surgical Landmark
Recently, the anterior rim of the incisura fibularis has been identified as a consistent intraoperative guideline. When performing open reduction and internal fixation of an ankle fracture, the anterior lip can be visualized, and thus proximal fibular positioning can be directly assessed. Sometimes, under fluoroscopy alone, a mild malreduction- especiallyrotational-can be overlooked.
Relying on the anterior end of the incisura as a feel and sight sensation provides a more reliable reductionwhich will ultimately result in improved patient effects
Implications for Treatment, Rehabilitation, and Prognosis
Knowledge of the anatomy of the notch guides rehabilitation. The shallowest incisura fibularis in a patient have them requiring a more aggressive program of ligamentous strengthening to restore stability at the ankle on recovery, because they have a lower native bony constraint. The type of fixation (syndesmotic screw, suture -button device, other construct) may also be some of the decision to act on the difference in notch morphology.
It has been shown long term function of the ankle after syndesmotic injury is determined by the accuracy with which the fibula is reduced into the notch making this trivial groove one of the most important details in ankle surgery.