Fig. 16. Ulnar styloid impaction. Frontal radiograph of the wrist demonstrates a curved, or ''parrot beak,'' ulnar styloid, with subchondral changes in both the ulnar styloid and the trique-trum (arrows). Soft tissue swelling at the ulnar aspect of the wrist is also seen.
(Fig. 18) [63-69]. Ulnar styloid nonunion is classified as type I or type II. Type I nonunion involves the tip of the ulnar styloid. The TFCC remains intact, and the DRUJ is stable. Type II nonunion involves the base of the ulnar styloid. Hence the TFCC attachment is disrupted, and the DRUJ is unstable . Conventional radiographs demonstrate nonunion of an ulnar styloid fracture and may show subchondral changes in the proximal triquetrum. MRI may show marrow edema, chondromalacia, and ununited bone fragments (Fig. 19). MR arthrography, as mentioned previously, can aid in better evaluation of the ulnar attachments of the TFCC . Treatment depends on the type of nonunion. For a type I nonunion, the bone fragment should be removed. For type II, the ul-nar styloid, along with the TFCC, should be fixed to the distal ulna .
Fig. 18. Ulnar styloid impaction with styloid nonunion. Coronal Tl-weighted (A) and fat-suppressed T2-weighted (B) images demonstrate a type 2 styloid nonunion (asterisk), with subchondral change in the ulnar aspect of the triquetrum (black arrowhead), subchondral change and marrow edema in the distal ulna and in the styloid fragment (white arrowheads), and surrounding soft tissue edema.
The normal lunate has one distal articular facet, where it articulates with the capitate. Approximately 44% to 73% of the population has a second facet at the ulnar side of the lunate, which articulates with the proximal pole of the hamate (Fig. 20) . This type of lunate has been called a type II lunate [70-72]. In a study of cadaveric wrists, Viegas and colleagues  found that the
incidence of arthrosis at the proximal pole of the hamate was 38.2% in the cadaver population with a type II lunate, compared with 1.8% in those with a type I lunate. In the same study, the authors found that the proximal pole of the hamate is one of the most frequent sites of cartilage damage in the wrist. This finding is most likely due to the disruption of the second carpal arc that occurs in patients with a type II lunate . During forced ulnar deviation, the hamate ''jumps'' over this disruption, leading to increased load on the proximal pole of the hamate . Many of these lesions are clinically silent, and correlation must be made with a patient's symptoms. A type II lunate may be seen on conventional radiographs with a reported accuracy of 64% to 72% . MRI and MR arthrography with a midcarpal joint injection may show chondromalacia of the proximal pole of the hamate, subchondral changes, and marrow edema [38,75].
Ulnar impingement syndrome is a condition that develops when a short ulna impinges on the distal radius proximal to the sigmoid notch [76,77]. The short ulna may be congenital, due to premature fusion of growth plates, or a result of prior surgery (eg, Darrach ulnar shortening surgery, Madelung corrective surgery, surgery for rheumatoid arthritis) [55,76]. The short ulna impinges on the distal radius, forming a painful pseudarthrosis, which is aggravated by pronation and supination (Fig. 21) . Conventional radiographs demonstrate a short ulna abutting the distal radius, with scalloping of the cortex of the distal radius proximal to the sigmoid notch . MRI can aid in earlier diagnosis, with detection of marrow edema and subcortical marrow changes.
Guyon's canal is a fascial tunnel that is bounded dorsally by the pisohamate ligament, volarly by forearm fascia and expansions from the flexor carpi ulnaris, medially by the pisiform, and laterally by the hook of the hamate. The canal extends approximately 4 cm from the proximal aspect of the pisiform to the level of the hamate. The ulnar nerve, ulnar artery, and (in some patients) veins pass through Guyon's canal . Any mass in this region could cause compression of the deep motor branch of the ulnar nerve. Masses that have been described in this region include ganglia, anomalous muscles, lipomas, and ulnar artery aneurysms .
Along with clinical history and physical examination, imaging is vital to narrowing down the differential diagnosis in ulnar-sided wrist pain. Treatments vary widely, depending on the cause of the pain, and differentiating the various entities is crucial. Conventional radiographs, conventional arthrography, CT, MRI, and MR arthrography are all useful modalities that are often used in concert to help guide diagnosis and treatment.
The author would like to thank D. Laurie Persson for his excellent original illustrations used in this review.
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