Psoas Major

Origin - two layers, and the plexus between them

• Superficial (anterior) layer: the anterolateral surfaces of the vertebral bodies and the intervening intervertebral discs of T12 to L4, together with the tendinous arches that bridge the constricted parts of the bodies, beneath which the lumbar arteries and veins and the sympathetic rami communicantes pass.
• Deep (posterior) layer: the anterior surfaces and lower borders of the transverse processes of L1 to L5.
• The lumbar plexus is formed in the plane between these two layers. The nerve roots emerge from the intervertebral foramina, pass behind the superficial layer and in front of the deep layer, and unite within the muscle substance.

Course

Descends along the pelvic brim, crossing the sacroiliac joint and the superior pubic ramus at the iliopectineal eminence, where it lies in a shallow groove between the eminence and the anterior inferior iliac spine. It then passes beneath the inguinal ligament in the lacuna musculorum, lateral to the iliopectineal arch, and lies directly on the anterior hip capsule before turning to its insertion.

Insertion

• A strong tendon onto the lesser trochanter of the femur, shared with iliacus as the iliopsoas tendon.
• Iliacus fibres attach onto the lateral side of the psoas tendon and continue directly onto the femur just distal to the lesser trochanter, so the composite unit is not a single tendon but a tendon with a muscular sleeve on its lateral aspect.
• At the level of the hip joint and labrum the unit is approximately 44.5 per cent tendon and 55.5 per cent muscle belly; at the lesser trochanter it is predominantly tendon. This gradient is the whole reason release level matters.

Psoas minor

A separate, slender muscle present in roughly 40 to 60 per cent of people. Arises from the bodies of T12 and L1 and the disc between them and inserts by a long thin tendon into the pectineal line and the iliopubic eminence, blending with the iliac fascia. It has no action on the hip - it is a weak flexor of the lumbar spine - and it is a frequent source of confusion at operation because it lies on the anterior surface of psoas major.

Psoas major

• Direct muscular branches from the ventral rami of L1, L2 and L3, sometimes with an L4 contribution. These enter the muscle segmentally from its posterior aspect.
• It is not supplied by the femoral nerve. This is the classic distinction from iliacus.

Iliacus (for comparison)

• Supplied by the femoral nerve (L2, L3), with the branches leaving the nerve within the abdomen, in the iliac fossa, proximal to the inguinal ligament.

Psoas minor

• L1, occasionally with L2.

The lumbar plexus formed within psoas - branch by branch

The single sentence to remember

Femoral nerve lateral, obturator nerve medial, genitofemoral nerve straight through the front.

Named vessels

• Lumbar arteries (from the abdominal aorta) - segmental branches passing beneath the tendinous arches of the origin to enter the muscle posteriorly. These are the vessels encountered and coagulated during a lateral transpsoas dissection.
• Iliolumbar artery (from the posterior division of the internal iliac artery) - its lumbar branch supplies the lower psoas and the iliacus.
• Branches of the common and external iliac arteries at the pelvic brim.
• Medial femoral circumflex artery and the obturator artery contribute to the distal muscle and the tendon near the lesser trochanter.

Vessels that matter as hazards, not as supply

• Segmental lumbar veins are thin-walled, unnamed and unforgiving; troublesome bleeding during a transpsoas approach usually comes from a venous plexus rather than an artery.
• The common iliac vessels and the vena cava lie anteromedially to the muscle. In lateral interbody surgery at L4-L5, the iliac vessels are the anterior limit of a safe working zone just as the plexus is the posterior limit - the corridor is bounded on both sides.
• Ascending lumbar vein joining the common iliac vein near the L4-L5 disc is the classic vessel avulsed during an anterior or lateral lumbar exposure.
• The iliolumbar vein and the corona mortis (an anastomosis between the obturator and external iliac or inferior epigastric systems, running over the superior pubic ramus) are relevant when the dissection extends to the pelvic brim.

Venous drainage

To the lumbar, iliolumbar and common iliac veins.

Anterior (from medial to lateral, in the abdomen)

• Kidney (its lower pole rests on the muscle), ureter (crossing anteriorly and then over the bifurcation of the common iliac artery), gonadal vessels, genitofemoral nerve on the muscle's surface, colon (ascending on the right, descending on the left), and the peritoneum.
• Aorta and inferior vena cava medially, sympathetic trunk on the medial edge over the vertebral bodies.

Posterior

• Transverse processes of the lumbar vertebrae, quadratus lumborum laterally, and the lumbar plexus within the muscle itself.

Medial

• Vertebral bodies, sympathetic trunk, aorta and vena cava, lumbosacral trunk and obturator nerve emerging at the brim.

Lateral

• Quadratus lumborum above, iliacus below, with the femoral nerve in the groove between psoas and iliacus.

At the hip

• The tendon lies directly on the anterior hip capsule at the 2 to 3 o'clock position of the acetabulum in the arthroscopic view, separated from it by the iliopsoas bursa, which is the largest bursa in the body and communicates with the hip joint in a minority of people, classically quoted as around 15 per cent.
• Femoral nerve, artery and vein lie anterior and medial to the tendon in the femoral triangle; the nerve is the most lateral of the three and is separated from the tendon by only a few millimetres of fascia.

Structures at risk

• Lumbar plexus within the muscle; genitofemoral nerve on its front; femoral nerve at its lateral border and in the femoral triangle; ureter anteriorly; iliac vessels anteromedially at L4-L5; peritoneum and bowel in a retroperitoneal approach.

Primary versus secondary

• Primary (haematogenous): more common in children, in the immunosuppressed, in diabetics and in intravenous drug users, and commoner in tropical and low-resource settings. Staphylococcus aureus is the dominant organism.
• Secondary (contiguous spread): from Crohn's disease, diverticulitis, appendicitis, urinary tract or renal infection, vertebral osteomyelitis and discitis, septic arthritis of the hip or sacroiliac joint, or infected spinal instrumentation. Organisms reflect the source and are frequently polymicrobial or enteric.
• Tuberculous psoas abscess is the classic form. In Pott's disease of the thoracolumbar spine, a cold abscess tracks down within the psoas sheath and may point below the inguinal ligament in the femoral triangle, presenting as a fluctuant groin swelling with no local signs of inflammation. The sheath is a closed conduit from the spine to the thigh - the anatomical explanation for a presentation that otherwise makes no sense.

Presentation

• The classic triad of fever, back or flank pain and a limp is present in only a minority of patients; the presentation is characteristically insidious and occult, and diagnostic delay drives the mortality and morbidity.
• Hip held in flexion with pain on passive extension (the psoas sign); pain referred to the groin, hip or knee.
• Insidious onset, weight loss and night sweats in tuberculous disease.
• A high index of suspicion is required - the diagnosis is frequently delayed, and delay is the main driver of morbidity.

Investigation

• Contrast-enhanced CT of the abdomen and pelvis is the investigation of choice: it demonstrates the collection, its extent, and usually its source.
• MRI is superior for spinal involvement and for distinguishing an abscess from a tumour.
• Blood cultures, inflammatory markers, and image-guided aspiration for microbiology including mycobacterial culture before starting antibiotics wherever the patient is stable.
• Look for the source in every case - a psoas abscess with no obvious cause needs the bowel, spine, urinary tract and hip examined.

Management

• Image-guided percutaneous drainage plus targeted antimicrobial therapy is first-line for most collections.
• Open drainage for multiloculated collections, failure of percutaneous drainage, or where the source requires surgical treatment.
• Treat the source: bowel disease, spinal infection, or an infected implant.
• Antituberculous chemotherapy where tuberculosis is confirmed or strongly suspected; drainage is often still required for a large collection but the mainstay is medical.

Internal snapping hip (coxa saltans interna)

A painful, audible or palpable snap arising as the iliopsoas tendon flicks over a bony or capsular prominence as the hip moves from flexion, abduction and external rotation into extension, adduction and internal rotation.

Where it snaps

• Most commonly over the iliopectineal eminence of the superior pubic ramus.
• Over the anterior hip capsule and the femoral head.
• Over a prominent anterior acetabular component after arthroplasty.
• Over the lesser trochanter itself, or over an exostosis or a fracture callus.
• Some tendons are bifid or accessory, and the snap arises from one slip catching on another - a described mechanism and a reason for failed release if the second slip is missed.

Who gets it

Dancers (up to a large proportion of professional ballet dancers report the snap), gymnasts, footballers and runners. Painless snapping is common and physiological; only pain and functional limitation warrant treatment.

Diagnosis

• Dynamic ultrasound is the investigation of choice and the only one that demonstrates the snap in real time.
• MRI or MR arthrography to exclude a labral tear, femoroacetabular impingement or a bursal collection.
• Diagnostic ultrasound-guided injection of local anaesthetic into the iliopsoas sheath - relief supports the diagnosis and predicts response to release.

Management

1. Non-operative: activity and technique modification, iliopsoas and anterior capsule stretching, eccentric hip flexor work, and a corticosteroid injection into the sheath. The majority settle.
2. Surgical release for the refractory painful snap:
   • Arthroscopic transcapsular release at the level of the hip joint - through the anterior capsule at the 2 to 3 o'clock position, or from the peripheral compartment. At this level only about 45 per cent of the unit is tendon, so a complete tendon release still leaves the iliacus muscle fibres intact, preserving more hip flexion power.
   • Release at the lesser trochanter - endoscopic or open. Here the unit is predominantly tendon, so the strength loss is greater; reserve it for a snap that clearly localises distally.
   • Always look for and address a second tendon slip.
3. Counselling: transient hip flexor weakness for 3 to 6 months is expected; recurrence and persistent weakness are the two failure modes.

Iliopsoas impingement after total hip arthroplasty

A recognised and under-diagnosed cause of persistent groin pain after hip replacement, occurring in a small but significant percentage of cases.

Mechanism and causes

The tendon runs directly over the anterior acetabular rim. Impingement follows anything that makes that rim prominent:

• An acetabular component that overhangs the anterior wall, usually from insufficient anteversion, excessive component size, or an anteriorly uncovered cup in a dysplastic or retroverted acetabulum.
• Prominent or retained cement, prominent acetabular screws, or a protruding screw hole plug.
• A large-diameter femoral head or an elevated liner.
• Excessive femoral offset or leg lengthening tightening the tendon.
• After hip resurfacing, a prominent uncontained cup edge.

Presentation

• Groin pain on active hip flexion - rising from a chair, climbing stairs, getting into or out of a car, and putting on shoes and socks.
• Pain on resisted straight leg raise and on resisted seated hip flexion, with a normal passive range.
• An audible or palpable snap in some patients.
• Onset is typically within the first months after surgery and does not settle.

Investigation

• Exclude infection and loosening first: inflammatory markers, radiographs, and aspiration if there is any doubt.
• CT with metal-artefact reduction to measure the anterior overhang of the acetabular component relative to the anterior wall - the single most important measurement.
• Ultrasound to show tendon thickening, bursal fluid and dynamic impingement.
• Ultrasound-guided local anaesthetic injection into the iliopsoas sheath - relief is both diagnostic and predictive of the benefit of tenotomy.

Management

1. Non-operative: activity modification, physiotherapy avoiding provocative resisted flexion, and an image-guided corticosteroid injection. A proportion settle.
2. Iliopsoas tenotomy (arthroscopic, endoscopic or open) for the patient with a small anterior overhang or who is medically unsuited to revision. Effective for pain in the majority, with the expected flexion weakness.
3. Acetabular component revision where a specific mechanical cause is identified - an over-long screw, cement extrusion, a screwed cup, or a grossly oversized or malpositioned component. In the original prospective series cure was achieved without changing the cup in 7 of 9 patients, including most of those who did have an overhanging cup, so radiographic overhang is not by itself an indication to revise. Reserve revision for the younger, higher-demand patient with a clearly identified implant abnormality.
4. Remove prominent screws or cement where those are the cause.

Iliopsoas bursitis

The iliopsoas bursa is the largest in the body, lying between the tendon and the anterior capsule, and it communicates with the hip joint in a minority of people, a figure classically quoted as around 15 per cent. A distended bursa can present as a groin mass, femoral vein compression with leg swelling, femoral nerve compression, or bowel or bladder symptoms. Causes include inflammatory arthritis, osteoarthritis with a joint effusion decompressing into the bursa, and impingement after arthroplasty.

Iliopsoas tendinopathy and avulsion

• Tendinopathy presents as anterior groin pain on resisted flexion, common in dancers and footballers.
• Avulsion of the lesser trochanter is an apophyseal injury in the adolescent athlete (sudden forceful flexion) and, in an adult with no significant trauma, is a pathological fracture until proven otherwise - most often a metastasis. That is a well-recognised examination trap.

Psoas and the hip-spine relationship

Psoas contracture forces anterior pelvic tilt and lumbar hyperlordosis; a stiff or fused lumbar spine removes pelvic compensation and increases the dislocation risk after total hip arthroplasty. Assessing spinopelvic mobility with standing and seated lateral radiographs before hip replacement is now standard practice in many units for this reason.

Psoas as a marker of sarcopenia

Cross-sectional area of psoas major on axial CT is used as a surrogate measure of sarcopenia and frailty, and correlates with outcome after major surgery and after hip fracture. It is a convenient measurement because it is available on almost every trauma CT.

The problem

The lateral retroperitoneal transpsoas approach reaches the lumbar disc by splitting psoas major - and therefore by passing directly through the territory of the lumbar plexus, which lies between the muscle's two layers.

The anatomical rule

The plexus is not centred in the muscle. At the upper lumbar levels it lies dorsally within psoas, close to the posterior third of the vertebral body. As you descend, it migrates ventrally, so that at L4-L5 it lies much closer to the middle of the disc. Simultaneously the iliac vessels and the genitofemoral nerve limit the corridor anteriorly. The safe corridor therefore narrows and moves anteriorly as you go caudally.

The working zones

Cadaveric and imaging work divides the lateral surface of the disc into quarters from anterior to posterior. The consistent findings are:

• L1-L2 to L3-L4: the safest docking position is the middle to posterior part of the disc (the mid-posterior quarter), because the plexus is still dorsal and the great vessels are the more anterior constraint.
• L4-L5: the safe position moves forward to the midpoint of the vertebral body - the demarcation between the mid-anterior and mid-posterior quarters (Zone II to Zone III) - because the plexus has migrated ventrally and the femoral nerve is now close to the middle of the disc space.
• L5-S1 is not accessible by this approach - the iliac crest blocks the lateral trajectory and the iliac vessels occupy the corridor.
• The genitofemoral nerve lies on the anterior surface of psoas and is at risk from a retractor blade placed too far anteriorly at any level.

Technique that protects the plexus

• True lateral decubitus position with the table broken to open the space between the iliac crest and the twelfth rib, and rigid fixation of the pelvis and thorax. Confirm true anteroposterior and lateral fluoroscopic images before docking - rotation is the commonest source of a mis-sited corridor.
• Retroperitoneal approach, sweeping the peritoneum forward digitally before touching psoas.
• Blunt, sequential dilatation through the psoas with continuous directional triggered electromyographic monitoring. Sharp dissection through the muscle is not acceptable.
• Interpret the thresholds: low stimulation thresholds indicate a nerve close to the probe; redirect anteriorly rather than proceeding. A high threshold in all directions is what permits docking.
• Minimise retraction time. Prolonged psoas retraction is an independent risk factor for neurological deficit; release the retractor intermittently in long cases.
• Do not place the posterior retractor blade behind the mid-disc at L4-L5, and do not place the anterior blade over the front of psoas where the genitofemoral nerve lies.

Expected versus serious neurological events

• Anterior thigh pain, numbness and hip flexor weakness are the rule, not an occasional complication, and are usually due to direct psoas trauma and to the genitofemoral or other sensory nerves rather than to the femoral nerve. Cummock's 59-patient series gives the numbers to quote: 62.7 per cent had thigh symptoms, and at first follow-up pain affected 39.0 per cent, numbness 42.4 per cent, paraesthesiae 11.9 per cent and weakness 23.7 per cent. By three months these had fallen to 15.5, 24.1, 5.6 and 11.3 per cent, and more than 90 per cent had resolved by one year. "Usually settles" is true; "uncommon" is not.
• True femoral nerve injury with quadriceps weakness and loss of knee extension is far less common but is functionally serious and may not fully recover. It is most frequent at L4-L5.
• Obturator, ilioinguinal and iliohypogastric injuries are described.
• Bowel injury, ureteric injury and vascular injury are rare but reported; the ureter is swept forward with the peritoneum and must not be mistaken for a vessel.

Consent

Name thigh numbness, hip flexor weakness and the small risk of a lasting quadriceps deficit explicitly. These are common enough that failing to warn is indefensible.

Why the femoral nerve is at risk

The femoral nerve emerges at the lateral border of psoas and travels in the groove between psoas and iliacus, immediately deep to the iliac fascia, before passing under the inguinal ligament within the lacuna musculorum lateral to the femoral vessels. In the Smith-Petersen anterior approach and in the direct anterior approach, the anterior acetabular retractor is placed over the anterior wall - and it sits directly on the iliopsoas, with the femoral nerve behind it.

The mechanism of injury

• Compression of the nerve between the retractor and the anterior acetabular wall or the superior pubic ramus. This is the dominant mechanism and is a function of retractor position, force and time.
• Traction from excessive femoral elevation and external rotation.
• Direct injury during anterior capsulectomy or during removal of anterior osteophytes.

How to avoid it

• Place the anterior retractor directly on bone, over the anterior acetabular wall, not into soft tissue, and use a blunt, broad-tipped retractor.
• Limit the depth of the retractor and check its position by feel; a retractor that has slipped medially off the anterior wall is sitting on the nerve.
• Release the anterior retractor periodically in a long case.
• Do not place a retractor over the superior pubic ramus medial to the iliopectineal eminence.
• Be aware that the direct anterior approach also risks the lateral femoral cutaneous nerve in its superficial dissection between sartorius and tensor fasciae latae - transient anterolateral thigh numbness is very common, and staying on the tensor fasciae latae side of the interval reduces it.

Other approaches

• Ilioinguinal approach to the acetabulum: the iliopsoas and femoral nerve occupy the middle window, between the iliopectineal fascia and the external iliac vessels. The iliopectineal fascia must be divided to develop the window, and the femoral nerve is retracted with the iliopsoas.
• Anterior intrapelvic (modified Stoppa) approach: works medial to the iliopsoas, and the corona mortis over the superior pubic ramus must be identified and ligated.
• Psoas tenotomy in cerebral palsy and in fixed flexion deformity: intramuscular lengthening of the psoas over the pelvic brim preserves the iliacus and produces less weakness than complete tenotomy at the lesser trochanter.