Saturday, December 18, 2010

Shoulder Instability and Upside Down Pears

Have you ever seen someone that reminds you of someone else? Above is a striking comparison between Gordan Freeman from Half-Life and Dr. House from the TV show House. There is even a website devoted to this topic, that you may find amusing. Doctors are no different, and when they see something in the body that looks like something else, they will often point that out, as we will see below.

The shoulder joint is composed primarily of the humeral head (red arrows), which fits into the glenoid portion of the scapula (blue arrows):

The glenoid portion of the scapula is best seen looking at the shoulder from the side, with the humeral head removed:

The shape of the glenoid can be compared to a pear:

The normal pear shape of the glenoid helps keep the humeral head in place. With repeated anterior dislocations, bone along the anterior margin of the glenoid can become worn down, and the glenoid can lose its normal shape.
The curve of the anterior glenoid becomes flattened, and the loss of the normal convexity of the glenoid can be part of the syndrome of recurrent anterior instability. The normal pear shape of the glenoid is lost; when the inferior part of the glenoid appears narrower than the superior part, one is said to have an "inverted-pear" glenoid.

In this case, a 29 year old male with a history of multiple left shoulder anterior dislocations was sent for an MR arthrogram. An axial intermediate image shows a large Bankart lesion:

A sagittal T1 weighted image reveals extensive flattening of the anterior margin of the glenoid, with an inverted pear appearance:

A CT scan shows the loss of the normal cup shape of the left glenoid on axial images (red arrow), with maintenance of the normal glenoid shape (green arrow) in the asymptomatic right shoulder:

A 3D volume rendered CT image better depicts the abnormal shape of the left glenoid:

For comparison, here is the shape of a normal glenoid:

Joe de Beer and his colleagues have recently written an excellent article on anterior shoulder instability in the elite athlete, with specific reference to bony deficiencies (Shoulder & Elbow 2010 2, pp 63–70). They note that:
--->Diagnosis of anterior glenoid bone loss can be missed at arthroscopy if one looks examines the glenoid solely from the posterior portal. This diagnosis is best made using the antero-superior portal.
--->Significant bone loss cut-off has been widely quoted as greater than 25%. This may not be the correct number in all patients: "What experience has taught us is that....the higher the demand, the less bone loss that is tolerated. We therefore deem significant even small bony deficiencies in the elite athlete".
Percentage bone loss can be measured on imaging studies (Griffith et al., AJR 2003;180:1423-1430) as well as at arthroscopy (Burkhart et al., Arthroscopy 2002; 18:488–91). The radiologist should alert the surgeon when there is appreciable bone loss along the anterior margin of the glenoid, as this information can alter the surgical procedure that is used to treat the patient.

Vic David MD