Bone outlines should be visualised, smooth and continuous:
- Iliopectineal/iliopubic line – Inner ilium and superior pubis.
- Ilioischial line – Inner ilium and medial ischium.
- Kohler’s teardrop – Summation of medial acetabulum, posterior acetabular wall.
- Anterior and posterior rims of acetabulum.
Effusion causes increased distance between teardrop of acetabulum and medial femoral head on unrotated AP. Bulging fat planes is not specific or sensitive.
Widened symphysis pubis
- Metastases, myeloma
- Cleidocranial dysplasia
- Epispadius, bladder extrophy, prune belly syndrome
Imaging Techniques and Applications
- AP pelvis – Internal rotation of hips .
- True/groin lateral hip
- Frog leg lateral – Obscures posterior portion of neck (by greater and lesser trochanters) and forshortens neck, same as AP.
- Sacroiliac joint views
- Judet views – 45deg oblique. Demonstrates ipsilateral posterior wall and ischium, contralateral anterior wall and pubis. LPO = left iliac/posterior oblique = right obturator/anterior oblique.
- Oblique pelvis views – 30deg oblique
- Prone symphysis
- anterior superior iliac spine (ASIS)
- Lesser trochanter
Consider CT in almost all acetabular fractures to check for free fragments, subtle fracture not seen on XR.
Angle between femoral neck and shaft is 135deg (115-140deg) and 15deg anteverted. Coxa valga is >140deg.
Symphysis pubic diastasis if >5mm in adults or >10mm in skeletally immature, or superoinferior offset >2mm. Sacroiliac diastasis if >4mm or asymmetric.
- Lateral compression injuries (most common) causes horizantal fractures of superior and inferior pubic rami
- Type I – Fracture medial acetabular wall without innominate bone rotation.
- Type II – Internal rotation of iliac wing, pubic and ischial fractures, disruption of posterior sacroiliac ligaments or fracture through posterior iliac wing/sacrum.
- Type III – Internal rotation of innominate bone and external rotation of contralateral bone. Risk of arterial haemorhage (type I-II low risk).
- Anteroposterior compression injury causes vertical fractures of superior and inferior pubic rami. Posterior acetabular fractures common.
- Type I – Vertical pubic rami fractures
- Type II (open book) – Symphysis pubis diastasis, disruption of anterior sacroiliac ligaments. Unstable, risk of arterial haemorrhage.
- Type III (sprung pelvis) – Diastasis of symphysis pubis and sacroiliac joints with disruption of anterior and posterior sacroiliac ligaments. Unstable, risk of arterial haemorrhage.
- Bucket handle fracture – Ipsilateral vertical fractures superior and inferior pubic rami with contralateral sacroiliac joint diastasis/vertical fracture.
- Straddle fracture – Bilateral superior and inferior pubic rami fractures from direct impact. Male urethral injury common.
- Vertical shear injury causes vertical displacement of a portion of pelvis.
- Malgaine fracture – Ipsilatearl pubic rami and sacroiliac disruptions. Unstable, highest risk of arterial haemorhage.
Pelvic ring classification:
- Class I – Isolated fractures that don’t disrupt the pelvic ring. Include apophyseal avulsion.
- Class II – Disruption of ring in one location, unusual unless associated with sacroiliac or symphysis pubis disruption. Includes iliac wing, sacral fracture, isolated ischial or pubic rami.
- Class III – Disruption in at lesast 2 locations.
- Class IV – Acetabular fracture.
Acetabulum divided in Y-shape with anterior/iliopubic and posterior/ilioischial load-bearing columns (hemipelvis). Anterior column includes iliopectineal line, anterior wall and pubic rami; posterior column includes ilioischial line, sciatic notch region and posterior wall.
Elementary fractures – If on a transverse slice an acetabular fracture plane is AP it is a transverse fracture; L-R through medial wall is column fracture; AP/oblique isolated to rims are wall fractures. Column fractures always extend through inferior pubic ramus/ischiopubic junction.
- Anterior column – Rare, through iliac wing, medial wall of acetabulum, acetabular floor, ischiopubic junction.
- Posterior colum – Common, through sciatic notch, medial wall of acetabulum, acetabular floor, ischiopubic junction.
- Spur sign – Spur of bone extending from sacroiliac joint posterosuperiorly to acetabulum on obturator oblique view, indicating posterior column or both column fracture.
- Transverse – Common, involving anterior and posterior columns at above or below acetabular roof, inferior pubic ramus is usually intact.
- Anterior and posterior (most common) wall fractures – Non-weightbearing lip/rim of acetabulum, don’t extend into iliac bones or inferior pubic ramus.
Associated/combination fractures (most common). Stellate fractures extend from central acetabulum with >/= 3 fragments.
- Both column
- T-shaped – Transverse with inferior component also involving inferior pubic ramus
- Transverse-posterior wall (most common)
- Anterior wall-posterior hemitransverse
- Posterior column-posterior wall
~1/2 of pelvic fractures. Interrupted and assymmetry of arcuate lines. May be obscured by bowel gas. Transverse process L5 fracture suggests occult sacral fracture.
Sacral insufficiency fracture – From osteoporosis or radiotherapy. Patchy/linear sclerosis in sacral ala usually mid S2-S3 ± cortical disruption (usually seen on CT). ‘Honda’/H sign on bone scan with bilateral stress fractures. Diffuse low T1 from oedema/haemorrhage.
Can mimic malignancy radiographically and histologically (healing causes high nuclear-to-chromatin ratio, high mitotic figure count). Common in long jumpers, sprinters, hurdlers, gymnasts, cheerleaders. Common sites include ischial tuberosity (hamstrings), ASIS (sartorius, tensor fasciae latae), AIIS (rectus femorus), iliac crest (abdominal muscles), greater trochanter (hip rotators gluteus medius/minimus, obturator, gemellus, piriformis), lesser trochanter (iliopsoas), body of pubis and inferior pubic ramus (adductors, gracilis).
Associated with femoral head impaction. Intra-articular bony fragments from acetabulum, avulsion of ligamentum teres or shear fracture-dislocation of femoral head. Risk of AVN is 50% if not reduced by 24hrs.
- Posterior (90%) – Femoral head posterosuperior, smaller (less magnification on AP), internally rotated (profiling greater trochanter and obscuring lesser). Commonly associated with posterior wall fracture.
- Obturator anterior dislocation – Flexed position with head antero-medio-inferior overlying obturator foramen.
- Iliac dislocation – Extended position, head anterosuperior with external rotation (lesser trochanter profiled).
Femoral neck fracture rare in young-middle aged adults. Incidence 10% women and 5% men at 80yo, 20% and 10% at 90yo. High associated with distal radius and proximal humerus fractures. May appear normal on XR, low threshold for MRI (can limit to T1 and STIR taking 10-15min), CT or bone scan (after 72hrs, sensitivity 90%). Generally the more proximal, the more displaced and higher risk of AVN and nonunion.
- Subcapital – Garden classifcation high complications with stages (nonunion, AVN). DDx ring osteophytes.
- Stage I – Incomplete with lateral impaction, valgus displacement. Femoral head trabeculae to acetabulum in varus alignment.
- Stage II – Complete without displacement, mild varus or anatomical. Trabeculae of head-acetabulum in valgus.
- Stage III – Complete with partial displacement varus angulation. Trabeculae of head-acetabulum in valgus.
- Stage IV – Complete with impaction/telescoping. Trabeculae of head-acetabulum anatomic.
- Midcervical (rare)
- Basicervical (rare)
Intertrochanteric fracture – Older population, less common than subcapital. 2-, 3- or 4- part fracture depending on lesser/greater trochanter involvement. Fracture line usually joints the trochanters, the opposite obliquity is rare and unstable. AVN and nonunion uncommon. Tx dynamic hip screw (prevents screw cutting-out through femoral head and allows impaction accelerating healing).
Avulsion fracture of lesser trochanter – In children and adolescents from avulsion of iliopsoas at lesser trochanter apophysis, pulled anterosuperiorly. In adults it is unusual if isolated, and an underlying lesion should be considered.
Slipped capital femoral epiphysis – see Paediatric Musculoskeletal
Femoral shaft fracture often comminuted with butterfly or segmental fractures. Tx in children cast. Tx in adults intramedullary nail with interlocking screws (doesn’t disrupt cortex where many muscles attach). Version/rotation measued by summing angles of femoral neck and condyles (between line at bottom edge of film and line through neck/post margins of condyles); should be within 5deg of contralateral side. This can be done on selected slices from scout.
Stress Reaction and Fractures
Stress fractures may progress to complete fracture and displacement with continued weight bearing. Associated with repetitive stress, but not always. May occur in the medial femoral neck proximal to lesser trochanter, medial prox/mid-shaft, posterior distal diaphysis of femur, proximal/middle/distal tibia. Usually horizontal to shaft, rarely vertical. Linear sclerosis, occasionally associated with aggressive periostitis. Pubic stress fracture usually at junction of pubis and ischium, common in long-distance runners. Thigh splints is similar to shin spints in the femur. Easier to see after 1-2/52, CT or MRI. Histologically may be confused with malignancy.
?Avulsion of adductor magnus. Aggressive-appearing at posterior medial epicondyle/supracondylar ridge of distal femur. May not be associated with pain. Increased uptake on bone scan. May have periosteal new bone formation (esp younger patients). Asymptomatic with rest.
Transient Regional Osteoporosis (TRO)
Painful self-limiting regional erythema and osteoporosis, middle-aged men and pregnant women. Most around the hip, but may involve any lower limb joint; upper limb unusual. Marked marrow oedema with increased bone scan uptake, osteopenia (may be severe at risk of fracture), periosteal reaction. May also involve acetabulum cf AVN. Cartilage preserved. Recovery within 10-12/12, but may reccur at same/different joint a few years later.
Femoroacetabular Impingement (FAI)
Impingment between acetabular labrum and articular cartilage of anterior femoral head-neck junction, leading to labral tear, cartilage wear and delamination, early OA. Diagnosis based on history, examination (provocation test 90deg flexion/adduction/internal rotation) and imaging. May be congenital or due to osteophytes, SUFE, Perthes disease, DDH (lateral rim syndrome), malunited fracture, acetabular protrusio. Subchondral marrow oedema (superior lateral acetabulum or lateral femoral head/neck), subchondral cysts, cartilage oedema/loss/delamination, labral tear, os acetabulare. Synovial herniation pit (fibrocystic change of the anterosuperior femoral neck, Pitt’s pit) – lucency with thin sclerotic border anterior/anterosuperior neck; controversial association (DDx impingement by anterior joint capsule or iliopsoas tendon).
- Cam type (femoral impingement) – Nonspherical head or bulge at anterolateral head-neck junction, impinging at flexion or internal rotation. Alpha angle >50deg – Between long axis of femoral neck and centre of femoral head to head-neck junction (where cortical surface extends outside perfect circle over the head). Blunted/upsloping lateral acetabular roof margin, small ossicles adjacent to acetabular rim.
- Pincer type – Excessively deep acetabulum with overgrowth of anterosuperior acetabular rim. Crossover sign with anterior margin of acetabulum projecting inferolateral to posterior margin on AP (posterior wall should always be more inferior).
- Mixed – Most.
Tx controversial ?resection of overgrowth, more important in younger patients with risk of early OA.
Arthritis and Inflammation
Reactive sclerosis on ilial side of sacroiliac joint after stress on the joint. Esp multiparous women. Symmetric, sclerosis triangular pointing inferiorly, no joint irregularity or involvement of sacral side. Asymptomatic, may resolve spontaneously.
Other Hip Conditions
Usually anterosuperior. Sulcus is normal variant at posteroinferior labrum. Best seen on MR arthrography.
Transient Osteoporosis of the Hip
Idiopathic (??early AVN), M>F, painful hip with no underlying disorder. Osteoporosis limited to the painful hip. MR similar to early AVN with low T1 throughout femoral head and neck. Self-limiting with full resolution.
Snapping Hip Syndromes
Snapping hip may be from iliotibial band or anterior gluteus maximus tendon over the greater trochanter, iliopsoas tendon over the pubic tubercle, labral tear, intraarticular body or femoroacetabular impingement.
Iliopsoas bursitis, infection
- Trochanteric bursitis – fluid lateral to greater trochanter.
- Iliopsoas bursitis
Medial margin of femoral head reaches or crosses line from lateral margin pelvic inlet to obturator foramen.
- Transient – Normal variant in preadolescent, doesn’t persist.
- Secondary – RA, OA, Turner’s, Marfan syndrome, pelvic fracture, bone softening (OI, Paget’s, fibrous dysplasia, osteomalacia, renal osteodystrophy).
- Primary (Otto pelvis) – Severe bilateral from malformation. F>M, familial. Usually associated with pain, obstetric complications, premature OA.
Nerve and Muscle Pathology
Gluteal pain in L5/S1 distribution from irritation of sciatic nerve as it courses between hip external rotator muscles posterior to femoral neck. From mass, piriformis muscle hypertrophy, trauma, aberrant sciatic nerve course through or around piriformis.
Partial tear of fascia around inguinal ligament. In athletes who run and twist waist (eg football, kickers).
Hip Joint Arthroplasty
Total joint replacement resurfaces both sides, for refractory pain in OA. Hemiarthropalsty usually for hip fracture at risk of AVN or nonunion. Most consist of metal alloy femoral/acetabular components lined with polyethylene plastic. May be fixed with methylmethacrylate (cement), screws or press fit (noncemented, porous surface to allow in-growth of bone for biological fixation) after reaming of native bone. Metal-on-polyethylene, metal-on-metal and ceramic prostheses have fewer particles, reducing loosening and particle disease. Radiotracer uptake expected for at least 1yr posterior surgery.
- Lateral opening (horizontal version) of acetabular cup – Angle between opening to transichial line should be 40 ± 10 deg; more at risk of dislocation, reduced limis abduction and risk of anterior dislocation.
- Acetabular anteversion – Should be 10-15deg, less if there is neck/shaft anteversion. Any retroversion at risk of posterior dislocation.
- Medial-lateral positionin – Centre of femoral head should be similar to normal head. If acetabulum to medial there may be excessive thinning of the wall and risk of fracture; too lateral means the iliopsoas tendon crosses medially forcing head from socket.
- Equal limb length – Comparing eg lesser/greater trochanters to contralateral side and transischial line. Shortening causes laxity of muscles and risk of dislocation; overlengthening stretches the neurovascular bundle and causes muscle spasm and dislocation.
- Appropriate size of components. Acetabular cups should have complete osseous coverage, femoral stems optimal proximal (cf distal) canal fit.
- Loosening – Migration of component (acetabular usually superomedial, femoral usually inferior = subsidence) or change in alignment, radiolucent zone (in cemented) >2mm is continuous around component or has progressively widened, fracture of cement or component, excessive cortical hypertrophy and endosteal bony bidging at tip of femoral stem.
- Calcar resorption – From stress sheilding of the femoral prosthesis, may be reduced with a calcar shelf (medial flange); one of the reasons why a revision arthropasty requires a longer stem for purchase into healthy bone.
- Prosthetic failure – Uncommon. Polyethylene insert separation from backing with small wedge-shaped metallic fragments, displaced polyethelene.
- Periprosthetic fractures – Femoral shaft usually from tip of prosthetic stem extending long anteriorly, usually nondisplaced.
- Polyethylene wear – Offset of femoral head within acetabuluar cup superolaterally (weightbearing portion). Major source of small particles.
- Particle disease (massive osteolysis) – Small (~size of RBC) particles (polyethylene, cement, osseous debris or metallic microspheres) induce granulomatous reaction and bone lysis; larger particles don’t incite this. Usually localised, scalloped margins.
- Infection – XR normal, mimic loosening or particle disease. Requires aspiration.