Compartment Anatomy of the Leg and Forearm: Four-Compartment and Three-Compartment Fasciotomy

Overview

Acute compartment syndrome (ACS) is a surgical emergency arising when intracompartmental pressure (ICP) rises to the point where local perfusion is compromised within a closed fascial space, producing progressive ischaemia and irreversible myonecrosis within 4-6 hours of complete ischaemia. The leg and forearm are the two most commonly affected sites. Definitive treatment is full-length open fasciotomy of all affected compartments, subcutaneous fasciotomy is contraindicated because skin itself acts as a limiting boundary and must be fully incised to allow muscle inspection and debridement.

Perfusion Threshold and Pressure Monitoring

The clinically actionable perfusion gradient is:

$$\Delta P = P_{\text{diastolic}} - P_{\text{compartment}}$$

A threshold of $\Delta P < 30\ \text{mmHg}$ is the widely accepted operative indication in equivocal or obtunded patients. In alert patients with unequivocal clinical signs, fasciotomy should not be delayed to obtain pressure measurements.

Pressure thresholds are guidelines only; clinical context (blood pressure, trend, patient reliability, mechanism) must always be integrated. Fasciotomy after 12 hours of established ACS is associated with substantially worse outcomes.

The Four Compartments of the Leg

High-Yield Anatomical Points

• The deep posterior compartment is the compartment most commonly missed at fasciotomy; it is most readily identified in the distal third of the leg where its fascial covering becomes superficial just above the ankle.
• The superficial peroneal nerve pierces the lateral compartment fascia approximately 10 cm above the lateral malleolus; ~75% remain within the lateral compartment to this point, while ~25% pass into the anterior compartment before exiting. This variability makes it the primary nerve at risk in the lateral fasciotomy.
• The saphenous vein and saphenous nerve run along the medial border of the leg and are at risk during the posteromedial incision.
• The posterior tibial artery and its perforating vessels supply posteromedial fasciocutaneous flaps; the medial incision is placed anterior to these perforators to preserve flap options.
• All four compartments must be released; current evidence does not support treating the anterior and lateral compartments as a single entity sufficient for complete decompression.

The Three Compartments of the Forearm

Anatomically, four compartments are described; clinically and surgically these are grouped into three decompression targets. The volar (superficial + deep) compartments are usually addressed through a single skin incision.

The "three compartments" descriptor groups the superficial and deep volar as a single surgical entity (accessed via one skin incision), with the mobile wad and dorsal compartment as separate targets, hence three functional decompression groups from four anatomical compartments.

Forearm Fasciotomy Incisions

Volar fasciotomy (performed first, required in virtually all cases):

• Curvilinear (S-shaped) incision beginning medial to the biceps tendon at the antecubital fossa
• Crosses the elbow flexion crease obliquely (lacertus fibrosus released proximally)
• Courses distally and ulnarwards, medial to palmaris longus
• Crosses the wrist flexion crease obliquely and continues into the midline of the palm for carpal tunnel release (nearly always required); curving ulnarwards at the wrist reduces risk to the palmar cutaneous branch of the median nerve
• Provides access to both superficial and deep volar compartments; the mobile wad can usually be released through radial extension of the same skin incision
• Pronator quadratus must be specifically examined and separately decompressed if needed

Dorsal fasciotomy (performed only if ICP remains elevated in the dorsal compartment after volar release):

• Straight longitudinal dorsal incision
• Released through the interval between ECRB and EDC

Indications for Fasciotomy

Most sensitive clinical sign: pain with passive stretch of muscles within the affected compartment. Late findings (5 Ps: paraesthesia, pallor, pulselessness, paralysis, poikilothermia) indicate advanced ischaemia. Preliminary measures (cast splitting with padding removal, dressing release, correction of hypotension, supplemental oxygen) may temporarily reduce ICP but must never delay fasciotomy once indicated.

Two-Incision Four-Compartment Leg Fasciotomy

Rationale for Two Incisions

Single-incision techniques (with or without fibulectomy) are described but are inferior because:

• Visualisation of the deep posterior compartment is incomplete, risking inadequate release
• Fibulectomy is unnecessarily destructive and risks the common peroneal nerve
• The two-incision technique provides superficial fascial access to all four compartments, is faster, and is considered safer

Incision 1: Anterolateral Incision

Position: Approximately 2-3 cm anterior to the fibula, over the intermuscular septum between the anterior and lateral compartments (roughly midway between the fibular shaft and the tibial crest).

Extent: From just below the fibular neck proximally (ensuring decompression of the muscle origins) to approximately 3-4 cm proximal to the lateral malleolus distally, full length of both compartments.

Steps:

1. Tourniquet applied but not inflated
2. Longitudinal skin incision; subcutaneous tissue elevated with wide undermining to expose fascia over both compartments
3. Transverse incision to identify the lateral intermuscular septum; superficial peroneal nerve identified just posterior to the septum
4. Anterior compartment fascia incised longitudinally in line with tibialis anterior using Metzenbaum scissors, proximally and distally
5. Lateral compartment fascia incised longitudinally in line with the fibular shaft, proximally and distally
6. Muscle viability assessed; non-viable tissue debrided

Critical hazard: The superficial peroneal nerve pierces the fascia ~10 cm above the lateral malleolus. It must be identified before the fasciotomy proceeds distally and explicitly protected.

Incision 2: Posteromedial Incision

Position: Approximately 1-2 cm posterior to the palpable posteromedial border of the tibia.

Rationale for position:

• Provides a generous skin bridge from the lateral incision
• Anterior to the posterior tibial artery and its perforating vessels, protecting fasciocutaneous flap territories
• Allows direct access to the superficial posterior compartment by skin retraction

Extent: Full length of the calf from the upper leg to just above the medial malleolus, ensuring the distal extent of the deep posterior compartment is decompressed.

Steps:

1. Longitudinal skin incision; wide subcutaneous undermining for exposure of fascial planes
2. Saphenous vein and nerve identified and retracted anteriorly
3. Superficial posterior compartment fascia (over gastrocnemius-soleus complex) incised longitudinally for the full compartment length
4. Transverse incision to identify the septum between superficial and deep posterior compartments
5. Soleus retracted posteriorly; deep posterior compartment fascia identified (most reliably in the distal third of the leg above the ankle) and incised longitudinally over FDL
6. If the soleus bridge extends more than halfway down the tibia, the soleus origin is partially elevated from the tibia to permit full-length deep posterior release
7. Peroneal vessels and posterior tibial structures protected throughout; muscle viability assessed and non-viable tissue debrided

Critical hazards (medial incision):

• Saphenous vein and saphenous nerve: retract anteriorly
• Posterior tibial nerve, artery, and vein: within the deep posterior compartment; gentle retraction of the superficial compartment is essential
• Perforating vessels from the posterior tibial artery, preserved to maintain fasciocutaneous flap vascularity

Summary Table: Two-Incision Leg Fasciotomy

Skin bridge: At minimum sufficient to prevent necrosis; the lateral incision must not be placed too posteriorly and the medial incision not too anteriorly.

Wound Management After Fasciotomy

Fasciotomy wounds must never be primarily closed acutely, premature closure re-elevates ICP. Skin closure or coverage should not be attempted unless all muscle groups are confirmed viable.

Complications

Outcomes

• Normal function restored in >90% of children when fasciotomy is performed promptly (Bae et al.)
• Normal function in 68% of adults when forearm fasciotomy performed within 12 hours of ACS onset
• Outcomes decline markedly when fasciotomy is delayed beyond 12 hours
• Incomplete four-compartment leg release is associated with significantly worse functional outcomes: return to sport ~67% vs ~91% for complete four-compartment release (Maher et al.)
• Volar forearm fasciotomy alone is sufficient in the majority of forearm ACS cases; dorsal release required only when dorsal ICP remains elevated

Paediatric Considerations

• Most common cause of forearm ACS in children: supracondylar humeral fracture
• Classical 5 Ps may be unreliable; the 3 As (agitation, anxiety, increasing analgesia requirement) are a validated early warning triad
• Full restoration of function in >90% of children with prompt fasciotomy
• Vessel-loop shoelace technique useful for gradual wound approximation prior to definitive closure

Sources

• AOA (Australian Orthopaedic Association) position statements
• AAOS guidelines (American Academy of Orthopaedic Surgeons)
• Cochrane Library reviews