OCCULT POST-TRAUMATIC AVASCULAR NECROSIS OF HIP REVEALED BY MRI

Department of Radiology, Musculoskeletal section, University of Miami, Miami, Florida, 33101, USA

ABSTRACT

Two hips with avascular necrosis had associated traumatic lesions noted on magnetic resonance imaging. Location of stress fracture in one femoral neck and acetabular contusion development prior to avascular necrosis favors these processes as being etiologic. These cases are particularly noteworthy in that non-displaced fractures or other musculoskeletal trauma not visible on plain films has rarely been associated with subsequent development of avascular necrosis.

KEY WORDS

Magnetic resonance, avascular necrosis, fracture, hip

INTRODUCTION

MRI is the preferred modality for evaluation of the painful hip when plain films are normal and there is clinical suspicion for avascular necrosis (AVN) or occult fracture^1-6. Fractures are both a common cause and a common result of AVN^5-8. Subcapital fractures of the femoral neck are especially prone to development of AVN, especially if the fracture is displaced or poorly reduced⁷. We report two cases of radiologically occult trauma to the hip revealed with MRI and likely accounting for the development of AVN of the femoral head.

CASE REPORTS

1. A 65 year old woman complained of right hip pain after a car accident. Plain films were normal and MRI performed 1 week after trauma showed evidence of superior acetabular T1 and T2 prolongation consistent with a bone contusion (Figs. 1a,1b). Biopsy was performed because of the remote possibility of neoplasm and results were negative. Patient had gradual decrease in pain over a four month period. Hip pain returned and was severe enough to warrant another MRI seven months after the initial trauma. Plain films at this time showed sclerosis of superior acetabulum. MRI showed AVN of femoral head (Figs. 1c,1d). Patient underwent total hip replacement which confirmed the presence of AVN with secondary degenerative changes.

2. An 81 year old woman with multiple medical problems complained of right hip pain. She had fallen down several times over the past year. Other than for a one week course of steroids, she had no predisposition for the development of AVN. Initial plain films were normal except for involutional osteoporosis (Fig. 2a). Because of worsening pain, MRI was performed 5 months later demonstrating subcapital stress fracture on the right and AVN (Figs 2b, 2c). Subsequent plain film showed collapse of right femoral head and sclerotic area in femoral neck consistent with healed stress fractures (Fig 2d). Patient underwent a total right hip replacement confirming the presence of AVN.

DISCUSSION

The pathogenesis of AVN is dependent on decrease in blood supply to the marrow. Femoral head blood supply originates from retinacular branches of the femoral circumflex arteries and the ligamentum teres artery⁵. Fractures of the femoral neck reduce blood supply by interrupting the retinacular vessels and capsular vessels. Hip dislocations reduce blood supply via ligamentum teres. The small artery within ligamentum teres is an increasingly important source of blood in the elderly hip⁷. This vessel, which is a branch of the obturator artery, might also be disrupted in direct acetabular trauma such as occurred in case one. No bony displacement was evident in either of our cases, ordinarily a factor mitigating against AVN development. In Gardner's series of femoral neck fractures, no patient with a nondisplaced fracture had this complication⁷.

The MRI depiction of hip pathology is largely dependent on low signal areas within otherwise normal high signal fatty marrow on T1-weighted images. Various patterns of AVN have been described⁵. Up to 80% of AVN cases will demonstrate a characteristic "double-line sign" on T2-weighted images, a feature present in the hip of case two (Fig. 2c)^1,6,9. The segmental distribution of abnormal signal, particularly in the anterosuperior aspect of subchondral femoral head is also useful for diagnosing AVN^4,6,8-10. AVN of the femoral head has rarely been shown to extend to the femoral neck. The fact that the "double line" usually demarcates the periphery of the lesion and that the femoral neck lesion is more amorphous supports the hypothesis that the femoral neck lesion is a process other than AVN, which we believe to be a stress fracture based on correlation with the plain films. Although the chronological relation of the AVN to the stress fracture cannot be proven, etiologically it is more likely that the AVN was the complication rather than the cause of the fracture.

MRI staging of AVN has been described and shown to be useful in the prediction of subchondral fracture development^6,10. In addition to the extent of signal abnormality, the center of the AVN lesion is least likely to have fracture when fat signal is present (stage A) and most likely to fracture when fibrous tissue is present (stage D). Intermediate stages have blood (stage B) and fluid signal (stage C). Based on this grading mechanism, both of our hips which were replaced were stage D. One disadvantage of MRI over plain films or CT in AVN evaluation is the limitation in demonstrating early femoral head collapse, a finding which often indicates the necessity for hip replacement¹⁰.

The role of MRI in diagnosis of traumatic conditions of the hip is less well established than that of diagnosing AVN. Bone bruise is a common entity, particularly about the knee, wherein there is a pattern of marrow edema similar to the pattern seen in the acetabulum of case one². Pathologically, trabecular fracture is suspected. Usually healing ensues with return of normal marrow signal. Occasionally healing with sclerosis or fibrosis, or secondary osteonecrosis may result, resulting in low signal persistence, as occurred in the acetabulum of case one.

Fractures tend to have a more linear morphology than bone contusion on MRI³. Location of abnormal signal in femoral neck would also favor fracture rather than AVN. Fractures will sometimes have a serpentine course, which if in a subcapital location might be difficult to distinguish from the "band-like" pattern of AVN^3,5. The subcapital fracture in case two has characteristic linear low signal surrounded on both sides by edema which has been described in the linear type of stress fracture as well as amorphous high signal edema^2,3. The subsequent plain films in case two are sufficiently characteristic for a healed stress fracture, to indicate that the signal abnormality on MRI in this area is due to the same process.

Increased sensitivity, specificity, and spatial resolution compared to bone scanning should promote MRI into the forefront of occult traumatic imaging. Our two cases, in addition, point out that occult trauma, can lead to AVN of the femoral head. This complication has rarely been attributed to trauma when there is no displacement. Perhaps the increased usage of MRI in problematic hip pain cases will reveal additional cases where occult traumatic conditions are revealed, particularly in older patients wherein a history of trauma is not clearly elicited.

REFERENCES

1. Coleman B.G.; Kressel H.Y.; Dalinka M.K.; Scheibler M.L.; Burk D.L.; Cohen E.K. Radiographically negative avascular necrosis: detection with MR imaging. Radiology 168:525-528; 1988.

2. Deutsch A.L.; Mink J.H. Magnetic resonance imaging of musculoskeletal injuries. Rad. Clin. N. A. 27(5):983-1002; 1989.

3. Deutsch A.L.; Mink J.H.; Waxman A.D. Occult fractures of the proximal femur: MR imaging. Radiology 170:113-116; 1989.

4. Markisz J.A.; Knowles R.J.R.; Altchek D.W.; Schneider R.; Whalen J.P.; Cahill P.T. Segmental patterns of avascular necrosis of the femoral heads: early detection with MR imaging. Radiology 162:717-720; 1987.

5. Gillespy T.; Genant H.K.; Helms C.A. Magnetic resonance imaging of osteonecrosis. Rad. Clin. N. A. 24(2):193-208; 1986.

6. Mitchell D.G.; Rao V.M.; Dalinka M.K.; Spritzer C.E.; Alavi A.; Steinberg M.E.; Fallon M.; Kressel H.Y. Femoral head avascular necrosis: correlation of MR imaging, radiographic staging, radionuclide imaging, and clinical findings. Radiology 162:709-715; 1987.

7. Garden R.S. Low angle fixation in fractures of the femoral neck. J Bone Joint Surg 43B:647-663; 1961.

8. Lang P.; Jergesen H.E.; Moseley M.E.; Block J.E.; Chafetz N.I.; Genant H.K. Avascular necrosis of the femoral head: high-field-strength MR imaging with histologic correlation. Radiology 169:517-524; 1988.

9. Mitchell D.G.; Joseph P.M.; Fallon M.; Hickey W.; Kressel H.Y.; Rao V.M.; Steinberg M.E.; Dalinka M.K. Chemical-shift imaging of the femoral head: an in vitro study of normal hips and hips with avascular necrosis. AJR 148:1159-1164; 1987.

10. Mitchell D.G.; Kressel H.Y.; Arger P.H.; Dalinka M.; Spritzer C.E.; Steinberg M.E. Avascular necrosis of the femoral head: morphologic assessment by MR imaging, with CT correlation. Radiology 161:739-742; 1986.

FIGURE LEGENDS

1a. Sagittal MRI (SE 500/20) of right hip shows low signal of superior acetabulum consistent with acetabular contusion.

1b. Coronal MRI (SE 2000/90) shows increased signal same portion of acetabulum indicating marrow edema, such as from acute contusion. The horizontally oriented low signal line in the center of edema may even represent a non-displaced fracture line.

1c. Coronal MRI (SE 450/17) seven months later shows low signal in both acetabulum and in femoral head, indicating interval development of AVN of the femoral head.

1d. Coronal MRI (SE 2000/90) shows persistent low signal in same areas which correlated with sclerosis on plain films (not shown). There was femoral head collapse and the MR signal indicates the presence of Stage 4 AVN.

2a. Initial plain film of both hips is normal except for demineralization.

2b. Coronal MRI (SE 700/20) shows transversely oriented low signal areas in subcapital femoral neck consistent with stress fractures and low signal in right femoral head consistent with AVN.

2c. Coronal MRI (SE 2000/70) shows right joint effusion, low signal in femoral head consistent with stage 4 AVN, and abnormal signal oriented transversely in subcapital region indicating stress fractures. Femoral neck abnormality is ill defined and high signal with some linear low signal component indicating mixed pattern for stress fracture appearance.

2d. Follow up plain film shows collapse and sclerosis of right femoral head due to AVN and sclerosis of both subcapital regions indicating healed stress fractures (white arrows).