Saturday, June 21, 2008

Stellate Crease and the Supraacetabular Fossa

There are many fossae in the body, some of which include the digastric fossa (a depression on the inner surface of the mandible, giving attachment to the anterior belly of the digastric muscle), the hyaloid fossa (a depression in the front of the vitreous body, holding the lens of the eye) and the popliteal fossa ( the depression in the posterior region of the knee).

The word fossa comes from the Latin work for "ditch". The Fosse Way was a Roman road in England that linked Exeter in South West England to Lincoln in the East Midlands. It was probably called this from the ditch on either side of it. Many sections of the Fosse Way form parts of modern roads and lanes:

Fosse Way, from the top of Brinklow Castle, Warwickshire
The acetabular fossa (also known as the cotyloid fossa) is the nonarticular area in the medial aspect of the acetabulum. In this diagram, it is outlined by the pink arrows:

In some normal individuals, there is an additional fossa in the roof of the acetabulum, called the supraacetabular fossa. This fossa can be seen on high-resolution hip MRI scans. This area is present in a small percentage of normal, asymptomatic individuals, and should not be mistaken for an osteochondral lesion. Coronal intermediate and intermediate fatsat MRI images from a fifteen year-old female teenager, demonstrating the supraacetabular fossa:

The supraacetabular fossa is characteristically located at the 12:00 position of the acetabulum. The underlying bone marrow will be normal in signal. In some patients, it may be bilateral, although not always in a symmetric fashion.

Here is another example, from the right hip of a twenty-nine year old man:

The supraacetabular fossa is a pit in the vicinity of the stellate crease of the acetabulum but is separate from it (Richard Villar MD, personal communication). The stellate crease is a shallow puckering in the articular cartilage of the acetabulum:


© J Anat., 1997

In this arthroscopic image (Santori & Villar; J. Anat. 191:461-463, 1997) the acetabular fossa is at the bottom left and the margin of the acetabulum is at the top right. The stellate crease is outlined by the red arrows. The green arrow denotes a variably present groove ('iliopubic groove") that may be a result of incomplete fusion of the triradiate cartilage (Santori & Villar; J. Anat. 191:461-463, 1997).
The stellate crease is below the resolution of current clinical MRI scanners. The supraacetabular fossa is a separate, distinct entity, and can be detected using current MRI scanners. The supraacetabular fossa is a normal variant, and should not be mistaken for a pathologic condition.


Vic David MD
Orthoradiology.com

Friday, June 13, 2008

Hook of the Hamate Fractures

The first two years of medical school are spent cramming great gobs of information into a brain that has an alarmingly difficult time retaining it.

Much of the information is achingly boring, and sleep-inducing to the extreme. It's not unusual to see a poor medical student head down in a textbook, drool cascading over the page, defeated by sheer dryness of the material.

Photo by umjanedoan
Mnemonics are legion, as they help you retain some of this information less painfully. Many of these mnemonics have sexual overtones, the better to remember them by. One well-known mnemonic describes the carpal bones of the hand:

Some Lovers Try Positions That They Cannot Handle.

This Kama Sutra resonating gem has helped droves of medical student remember:

Scaphoid
Lunate
Triquetral
Pisiform
Trapezium
Trapezoid
Capitate
Hamate

The last bone on this list, the hamate, has a distinctive shape:


On axial MRI scans, the bone is oriented with the hook (blue arrow) facing down:


It should come as no surprise that the name of this bone is derived from the latin word for hook, hamus.

The hamate bone can be fractured one of two locations: the body or the hook.

Hook of the hamate fractures can be painful, and one must look for these carefully on MRI scans of the wrist. Many of these patients present weeks or months after injury, and bone marrow edema may be minimal or absent.

46 year-old female with wrist pain:


(A) Axial T1-weighted image reveals a fracture of the hook of the hamate. Fluid-sensitive images were also performed, but there was no edema associated with this fracture. (B) Comparison image from normal patient depicting normal hook of the hamate.

A common mistake on MRI is to look only at fluid-sensitive images to detect pathologic states. One should examine anatomic images such as T1-weighted, proton-density weighted and intermediate weighted images carefully as well, as some pathologic states are detected best on these images.

This patient had chronic pain related to this fracture. Once it was detected, the hook of the hamate was resected, curing the patient.

One pitfall to avoid is mistaking a bipartite hamate hook (os hamuli proprium) for a hamate fracture:


This accessory ossicle (green arrows) will often be oval or pyramidal in shape, and will typically be embedded in the pisohamate ligament (blue arrow).

Remember- look specifically at the hook of the hamate on axial MRI scans, or you may miss this fracture, particularly if the injury is subacute or chronic in nature.


Vic David MD
Orthoradiology.com