Iliacus Anatomy

Origin. The muscle arises from the inner surface of the ilium and the structures at its margins:

• The upper two-thirds of the concavity of the iliac fossa, the broad, smooth inner surface of the ilium
• The inner lip of the iliac crest
• The anterior sacroiliac and iliolumbar ligaments and the upper lateral part of the sacrum (the ala)
• A few fibres from the anterior inferior iliac spine and the upper part of the hip capsule, which blend with iliocapsularis

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Insertion. The fibres converge inferomedially onto the lateral side of the tendon of psoas major, forming the composite iliopsoas tendon, and some continue past the tendon to insert directly onto the femur just distal to the lesser trochanter, along the spiral line. The psoas tendon is the central cord that reaches the lesser trochanter; iliacus is a muscular sleeve on its lateral aspect, so the unit is a tendon with muscle wrapped around it and the tendinous proportion increases distally, approximately 40 per cent tendon at the level of the hip joint and predominantly tendon at the lesser trochanter.

That gradient is why the level of an iliopsoas release is a real clinical decision rather than a matter of preference. Releasing the unit at the level of the joint divides the tendon while leaving the iliacus fibres intact, and so preserves more hip flexion power than a release at the lesser trochanter; a tendon-only release at the joint does not abolish the muscle's action.

Course. The unit passes over the anterior hip capsule and the iliopectineal eminence, beneath the inguinal ligament in the lacuna musculorum, and turns posteriorly around the front of the hip to reach the lesser trochanter on the posteromedial femur.

Iliocapsularis (iliacus minor). A distinct and constant muscle in most individuals, though frequently overlooked. It arises from the anterior inferior iliac spine and the anterior aspect of the hip joint capsule, to which it is intimately adherent, and inserts on the femur just distal to the lesser trochanter along with the medial fibres of iliacus. Because its origin is the capsule itself, contracting it tensions the anterior capsule, and it acts as a dynamic anterior stabiliser of the hip, particularly where bony coverage is deficient.

Femoral nerve (L2, L3). Iliacus is supplied by branches of the femoral nerve, and the point that is examined is where those branches arise: within the abdomen, in the iliac fossa, proximal to the inguinal ligament. They enter the anterior surface of the muscle, usually as two or three twigs. Iliocapsularis is supplied by the same nerve, by branches to the medial part of the muscle.

The localising rule. Because the iliacus branches leave above the inguinal ligament, a femoral nerve lesion in the thigh paralyses quadriceps and produces saphenous sensory loss but spares iliacus, so hip flexion is preserved. A lesion in the pelvis paralyses iliacus and quadriceps together, so hip flexion is weak as well. This is the bedside test for the level of a femoral neuropathy, and it is the reason a retroperitoneal haematoma and a groin injury present differently.

Hip flexion is weakened but never abolished by a femoral nerve lesion at either level, because psoas major is supplied by direct muscular branches from the ventral rami of L1, L2 and L3 (sometimes L4) and not by the femoral nerve. The two halves of iliopsoas have different nerves and only one of them is affected; expecting complete loss of hip flexion leads to the wrong conclusion.

Named vessels. The dominant supply is the iliac branch of the iliolumbar artery, from the posterior division of the internal iliac artery, entering the muscle in the upper iliac fossa; it is the vessel that bleeds when the muscle is stripped from the inner table. The remainder come from several sources:

• Deep circumflex iliac artery (from the external iliac artery just above the inguinal ligament), running along the inner lip of the iliac crest and supplying the muscle and the crest; it is the pedicle of the vascularised iliac crest bone graft and must be preserved when that graft is planned
• Ascending branch of the lateral femoral circumflex artery, to the distal muscle near the hip
• Obturator artery, in some individuals
• Medial femoral circumflex artery, to the region of the tendon and the lesser trochanter

Venous drainage is to the iliolumbar, deep circumflex iliac and external iliac veins.

Vessels as hazards. The deep circumflex iliac vessels are divided or coagulated during anterior iliac crest harvest, and their pulsating branch on the inner lip is a reliable source of post-operative haematoma if not controlled. The superior gluteal artery, emerging from the greater sciatic notch, is at risk in posterior crest harvest if the dissection strays too far anteriorly and inferiorly; bleeding here retracts into the pelvis and is difficult to control from a posterior wound.

The corona mortis, an anastomotic vessel between the obturator and the external iliac or inferior epigastric systems, crosses the superior pubic ramus and is encountered in the medial windows of pelvic approaches.

The iliac fascia. A thick, tough sheet covers the anterior surface of iliacus, continuous with the psoas fascia medially and firmly attached laterally to the inner lip of the iliac crest and medially to the pelvic brim and the arcuate line. Inferiorly it thickens as the iliopectineal arch, running from the inguinal ligament to the iliopectineal eminence, and it continues into the thigh as part of the posterior wall of the femoral sheath. Because the fascia is unyielding and firmly anchored, the space beneath it behaves as a closed compartment: blood accumulating there cannot escape and compresses its contents.

Contents of the compartment. Deep to the fascia lie:

• Iliacus and psoas major
• The femoral nerve, in the groove between the two muscles, deep to the fascia
• The lateral femoral cutaneous nerve, in the upper lateral part
• The genitofemoral nerve, on the anterior surface of psoas, superficial within the same plane

Anterior and medial relations. In front of the fascia lie the extraperitoneal fat and peritoneum, with the caecum and appendix on the right and the sigmoid colon on the left; the ureter and the gonadal vessels cross anteriorly at the brim. Medially is psoas major, with the femoral nerve between the two muscles, and the external iliac vessels at the brim.

Distal relations. The unit passes over the anterior hip capsule, separated from it by the iliopsoas bursa, the largest bursa in the body, which communicates with the hip joint in a minority of people. Beneath the inguinal ligament the iliopectineal arch divides the space in two: the lacuna musculorum laterally carries iliopsoas, the femoral nerve and the lateral femoral cutaneous nerve, and the lacuna vasorum medially carries the femoral artery, femoral vein, lymphatics and the femoral canal.

The femoral nerve is not inside the femoral sheath. It lies lateral to the sheath, in the muscular compartment, which is why it is spared in a femoral hernia and why a femoral nerve block and a femoral vein cannulation are different targets.

The condition. Bleeding into the closed iliac fascia compartment compresses the femoral nerve against the iliac fossa. It is the classical compartment-type neuropathy of the pelvis.

Causes. The recognised settings are:

• Anticoagulation, with warfarin and the direct oral anticoagulants, the commonest current cause
• Haemophilia and other bleeding disorders, a classical presentation in haemophilia A and B and one of the recognised muscle bleeds along with the calf and forearm
• Blunt trauma and hyperextension injuries of the hip, including sporting injuries in young athletes
• After retroperitoneal or vascular procedures, femoral artery catheterisation and pelvic surgery
• Spontaneous, particularly in the elderly and in renal failure

Presentation. Pain in the groin, iliac fossa or back, with the hip held in flexion and slight external rotation and resisting passive extension, together with abdominal or iliac fossa tenderness. The femoral nerve deficit is progressive: quadriceps weakness, an absent knee jerk, and numbness over the anterior thigh and the medial leg and foot in the saphenous distribution. A falling haemoglobin and, in a large bleed, haemodynamic compromise complete the picture.

Symptoms may evolve over hours to days, and the neurological deficit typically follows the pain. Check the coagulation and image urgently.

Investigation. Urgent contrast-enhanced CT of the abdomen and pelvis demonstrates the haematoma, its size and any active extravasation; ultrasound can confirm a collection quickly at the bedside but is less complete. Send a full coagulation screen, factor assays where a bleeding disorder is suspected, and a group and save.

Management. Treatment proceeds as follows:

1. Correct the coagulopathy immediately: vitamin K and prothrombin complex concentrate for warfarin, the appropriate reversal agent for a direct oral anticoagulant, and factor replacement in haemophilia, which is the mainstay and is often sufficient by itself
2. Resuscitate and monitor serially, with documented neurological examinations
3. Conservative management is appropriate for most patients whose deficit is stable or improving after correction of the coagulopathy; recovery of the nerve is usual, though it may take months
4. Surgical or radiological intervention for a progressive neurological deficit, an expanding haematoma, haemodynamic instability, or failure of correction: angiographic embolisation of an identified bleeding vessel, or surgical decompression and evacuation with release of the iliac fascia through a retroperitoneal approach
5. Decompression is controversial in haemophilia, where factor replacement alone usually suffices and surgery risks further bleeding; decisions there are made jointly with haematology

The imaging measurement. The cross-sectional area of iliocapsularis on axial MRI or CT is usually expressed as a ratio against rectus femoris at the same level. A larger iliocapsularis indicates a hip that is relying on soft tissue for stability. It is a supporting measurement, used alongside the lateral centre-edge angle, the acetabular index, femoral version and the anterior wall index, and not a diagnosis by itself.

Why the distinction changes the operation. The decision between an acetabular reorienting osteotomy and an arthroscopic impingement procedure is genuinely difficult in the borderline hip, and getting it wrong is harmful:

• Dysplastic, unstable hip. The correct treatment is to restore bony coverage with a periacetabular osteotomy. An isolated arthroscopic labral debridement and capsulotomy in this hip removes the remaining dynamic and static anterior restraints and can precipitate frank instability or rapid degeneration, a well-recognised and serious error; recognising the hypertrophied muscle pre-operatively is one way to avoid it
• Over-covered pincer hip. The correct treatment is rim trimming and labral refixation, with capsular repair
• Borderline dysplasia, with the lateral centre-edge angle in the intermediate range, is the hardest group, and a hypertrophied iliocapsularis argues for treating the hip as unstable

In the operating theatre. Iliocapsularis is encountered as a distinct muscle overlying the anterior capsule in the direct anterior and Smith-Petersen approaches and in the arthroscopic peripheral compartment. Recognising it prevents it being mistaken for capsule and inadvertently divided.

Iliopsoas bursitis and tendinopathy. Iliacus shares the iliopsoas bursa and the distal tendon with psoas, and therefore shares bursitis, tendinopathy, snapping and post-arthroplasty impingement. A distended bursa can present as a groin mass with femoral vein or nerve compression.

Developmental dysplasia of the hip. In open reduction of a dislocated hip, the iliopsoas tendon crossing the medial capsule produces the hourglass constriction that prevents concentric reduction. Release or lengthening of the tendon over the pelvic brim is a routine part of the operation, along with excision of the pulvinar and the ligamentum teres and release of the transverse acetabular ligament.

Denervation. Denervation of iliacus with preserved psoas is seen with a femoral nerve lesion in the pelvis, from a retroperitoneal haematoma, a mass, radiation, diabetic amyotrophy, or surgical injury during pelvic or anterior spinal surgery. On MRI, selective iliacus denervation with a normal psoas localises the lesion to the femoral nerve rather than to the plexus or the roots.

The iliac fossa as a site of disease. The fossa is a site for pelvic bone tumours (chondrosarcoma, Ewing sarcoma), for retroperitoneal collections and abscesses tracking from the spine, and for transplant kidney placement, all of which distort or displace iliacus and alter the surgical anatomy.

Principle. The ilioinguinal approach (Letournel) exposes the anterior column and the quadrilateral surface through three windows created between the structures crossing the pelvic brim. Iliacus defines the first of them.

Lateral window. Created by subperiosteal elevation of iliacus from the internal iliac fossa, from the iliac crest down to the pelvic brim, it gives access to the internal iliac fossa, the anterior sacroiliac joint, the upper anterior column and the iliac wing; the same elevation gives access for the Bernese periacetabular osteotomy, the Salter innominate osteotomy and pelvic tumour resection. Its medial boundary is the iliopsoas with the femoral nerve, retracted medially. Strip strictly subperiosteally with a broad elevator, do not damage the iliac apophysis in an adolescent, and pack the fossa to control the diffuse bleeding from the muscle and the iliolumbar branches while the other parts of the exposure are developed.

Middle window. This lies between the iliopsoas with the femoral nerve laterally and the external iliac vessels medially, and is created by dividing the iliopectineal fascia, which separates the muscular from the vascular compartment and runs from the inguinal ligament to the iliopectineal eminence. It gives access to the pelvic brim, the quadrilateral surface, the superior pubic ramus and the anterior column. At risk are the femoral nerve, retracted with the iliopsoas; the external iliac vessels, which must be slung and handled gently; and the lymphatics, whose disruption causes a persistent lymphocele.

Medial window. Medial to the external iliac vessels and containing the spermatic cord or round ligament, it gives access to the superior pubic ramus and the pubic symphysis. The corona mortis crossing the superior pubic ramus must be identified and ligated before it is avulsed.

The modern alternative. The anterior intrapelvic (modified Stoppa) approach, often combined with the lateral window, works medial to the iliopsoas from a midline or Pfannenstiel incision and gives excellent access to the quadrilateral surface without the middle window dissection around the femoral nerve and the external iliac vessels. It has largely replaced the classical three-window ilioinguinal approach in many units.

Bernese periacetabular osteotomy. The approach, a modified Smith-Petersen or a direct anterior variant, involves elevating iliacus from the inner table to expose the quadrilateral surface and the ischium, and detaching or partially releasing the rectus femoris direct head from the anterior inferior iliac spine; iliocapsularis overlies the anterior capsule and is elevated with it. The lateral femoral cutaneous nerve is at risk in the superficial dissection, and many surgeons deliberately sacrifice or accept injury to it and warn the patient of anterolateral thigh numbness. The femoral nerve is protected by keeping retractors on the inner table and by limiting retraction time, and a femoral nerve palsy after periacetabular osteotomy is well described; the sciatic nerve and superior gluteal vessels are at risk during the posterior column and ischial cuts.

Salter innominate osteotomy and open reduction in dysplasia. In the child the iliac apophysis is split and iliacus reflected from the inner table with the periosteum, preserving the apophysis for closure; this is also how iliacus is stripped in the anterior (Smith-Petersen) exposure for open reduction, with the two halves of the apophysis reflected. Do not strip the abductors and iliacus circumferentially in the young child; preserve the apophysis and the periosteal sleeve.

Iliac crest bone graft harvest. An anterior harvest begins at least 2 to 3 cm posterior to the anterior superior iliac spine, which protects the lateral femoral cutaneous nerve and avoids an avulsion fracture of the spine through bone weakened at the sartorius and inguinal ligament attachment. Iliacus is elevated from the inner table subperiosteally, the deep circumflex iliac branches are controlled, and the inner table cortex is replaced or bone wax and a drain used to reduce haematoma.

A posterior harvest stays within 8 cm lateral to the posterior superior iliac spine, because beyond that the superior cluneal nerves cross the crest. It must not violate the sacroiliac joint medially, and must not dissect toward the greater sciatic notch where the superior gluteal artery lies.

Anterior approaches to the hip. In the Smith-Petersen and direct anterior approaches, iliacus and the iliopsoas tendon lie directly under the anterior acetabular retractor with the femoral nerve immediately behind them. Place the retractor on bone over the anterior wall, use a blunt broad tip, and release it periodically. The lateral femoral cutaneous nerve is crossed in the superficial interval; staying on the tensor fasciae latae side of the interval between sartorius and tensor reduces the incidence of anterolateral thigh numbness, which is otherwise very common.