10.5 Proximal Femur Problems: Ischium, Short Neck, Arthritis, Fracture, and Hardware

10.5 Proximal Femur Problems: Ischium, Short Neck, Arthritis, Fracture, and Hardware

Quick Answer: The femur's four named problem types are poor bone edge detection, nonremovable artifacts, ischium underlying the neck, and inaccurate BMC or BMD values. Their named causes are variant anatomy (short femoral neck, inadequate space between ischium and femur), fractures, pathology (arthritis, joint disease), and surgery (biomechanical devices). The femur's defining difficulty is that its most important ROI sits between two other bones.

Why the Femur Is Harder Than It Looks

In the lumbar spine, the confounding structures are mostly different from bone — calcification, metal, gas. In the proximal femur, the confounders are bone: the ischium medially, the greater trochanter laterally, the acetabulum superiorly. Edge detection separates bone from soft tissue by density, and it cannot separate bone from bone. That is the origin of most femur analysis failure.

Ischium Underlying the Femoral Neck

The ischial ramus lies medial and slightly posterior to the femoral neck. On a supine PA projection the two can overlap, and when they do, ischial mineral is added to the femoral neck ROI. The consequence is a falsely elevated femoral neck BMD.

Causes of Overlap

• Insufficient internal rotation, which leaves the neck oblique and shifts its projected position
• Wide pelvis or a naturally short distance between the ischium and the femur
• Hip adduction — the leg drawn toward the midline
• Pelvic obliquity from leg length discrepancy or a contracture
• Poor patient centering on the table

Responses

1. Reposition. Verify internal rotation, confirm the shaft is parallel and not adducted, and level the pelvis if it is oblique. In many patients this opens adequate clearance.
2. Verify the ROI boundary. Inspect whether the neck box, as placed, includes ischial bone. Adjust per the manufacturer's method.
3. Accept and document when anatomy is the cause. The ARRT outline names "inadequate space between ischium and femur" as variant anatomy for a reason — some patients simply have it, and it is not correctable. Document and reproduce.
4. Consider the contralateral hip, which may have better clearance.
5. Weight the total hip in interpretation, since it is less affected than the neck and has better precision.

Short Femoral Neck

A short femoral neck compresses the distance between the femoral head and the intertrochanteric region. The standard-width femoral neck box may not fit within it without overlapping the head superiorly or the trochanteric region inferiorly.

• Recognize it on the image: the neck appears stubby with little clear space between head and trochanter.
• Place the box at the narrowest portion of the neck, per the manufacturer's convention, even if the automatic placement drifts.
• Document the placement and reproduce it. This is a high-yield precision issue, because a short neck gives the box little room and small placement differences translate into large BMD differences.
• Coxa vara, developmental dysplasia, and prior slipped capital femoral epiphysis all produce short or abnormally angled necks.

Poor Bone Edge Detection at the Femur

Response is the same hierarchy as elsewhere: appropriate mode, low-density analysis options, inspect the bone map against anatomy, correct manually per the manufacturer's tools, document.

Pathology

Hip Osteoarthritis

Osteoarthritis is the femur's equivalent of lumbar degenerative disease, and it does two things at once:

• Raises apparent BMD through subchondral sclerosis at the femoral head and acetabulum, marginal osteophytes, and cyst walls. The total hip is more affected than the femoral neck, because the trochanteric and intertrochanteric regions lie closer to the degenerative changes.
• Prevents correct positioning, because a painful arthritic hip will not internally rotate.

So the same disease inflates the number and degrades the technique.

Other Pathology

• Avascular necrosis of the femoral head: sclerosis and collapse alter density.
• Paget disease: markedly elevated, unreliable values; exclude the site.
• Metastatic disease: lytic lesions lower and blastic lesions raise density; either invalidates the site.
• Heterotopic ossification after surgery or trauma: dense bone in the soft tissue field.
• Severe osteoarthritis of the contralateral hip may be why the patient cannot lie symmetrically.

Fractures

• Acute or healing proximal femur fracture: that femur is not measurable. Callus, deformity, and hardware all corrupt the result. Scan the contralateral hip.
• Healed fracture with retained hardware: unmeasurable.
• Healed fracture without hardware: deformity and remodeled bone still compromise validity; the contralateral hip is preferred, and if used, document.
• Atypical femoral fracture in long-term bisphosphonate users: a reason many facilities add a long femur view, which is a screening image rather than a BMD measurement.

Surgery and Biomechanical Devices

The Site-Selection Decision Tree

When femur problems arise, the fallback order follows ISCD's valid diagnostic sites:

1. The unaffected contralateral hip, if one hip is compromised.
2. The lumbar spine, if both hips are compromised.
3. The 33% radius, if the spine is also unmeasurable or uninterpretable, or in hyperparathyroidism, or when the patient exceeds table limits.
4. Report the limitation explicitly when no valid site remains.

And once a side is chosen, it is the side for the life of the patient's monitoring record. A baseline on the right and a follow-up on the left is not a comparison.