What causes pencil pointing of the distal clavicle on shoulder radiography?
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Answer:
Pencil pointing is progressive tapering of the lateral end of the clavicle after erosion has removed its cortical outline, so the bone ends in a sharpened point instead of a squared articular surface. In rheumatoid arthritis the process begins as resorption of the undersurface of the distal clavicle, an early and often overlooked erosive change described by Martel, and progresses to widening of the acromioclavicular joint and frank tapering. It is not specific, and the differential is a checklist of resorptive processes at that site: hyperparathyroidism, in which distal clavicular resorption accompanies subperiosteal resorption on the radial aspects of the middle phalanges and a salt-and-pepper skull; post-traumatic osteolysis and weightlifter’s clavicle, which are unilateral and follow repetitive load; scleroderma and multicentric reticulohistiocytosis; infection; and myeloma or metastasis producing focal destruction. The features that point to rheumatoid disease are the accompanying findings elsewhere in the shoulder and hand: marginal erosions, symmetric uniform joint space narrowing without osteophytosis, periarticular osteopenia, and superior migration of the humeral head with acromiohumeral distance under 7 mm indicating chronic rotator cuff tear. MRI and ultrasound show the synovitis and cuff disease that radiographs infer.
Why is it called so?
The bone end is whittled to a fine tip like a sharpened pencil.
Pathophysiology
The distal clavicle has a thin cortex, a substantial subchondral plate and a rich synovial covering at the acromioclavicular joint, which makes it a favoured site for pannus-mediated bone destruction. In rheumatoid arthritis, hypertrophic synovium at the joint margins and along the inferior capsular attachment produces cytokines that drive osteoclastic resorption, and the undersurface of the clavicle, where the capsule attaches and where the cortex is thinnest, erodes first. Continued circumferential resorption removes bone symmetrically from the periphery inward, so the remaining bone tapers. In hyperparathyroidism the same site is affected through osteoclastic subperiosteal resorption driven by parathyroid hormone, which is why the two conditions converge on the same radiographic appearance.
Alternative names: Tapered distal clavicle; penciling of the clavicle
Other associated named signs: Rat bite erosions, the pencil-in-cup deformity of psoriatic arthritis, and superior migration of the humeral head
References
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