Understanding the Injury Mechanism in Hamate Hook Fractures by Investigating Fracture Morphologies: A Case Series Study

Abstract

Background: Many studies have described hamate hook fractures resulting from direct force from sporting tools. However, several authors have reported fractures that did not occur during swing-related activities. This study aimed to understand the injury mechanism of fractures by investigating their morphologies.

Methods: We selected patients with hamate hook fractures and collected data on computed tomography scans, injury causes, and how athletes handled sporting tools.

Results: We investigated 50 patients, and the study cohort included 32 patients who sustained injuries during sports: 24 during baseball (group A) and 8 during other sports (group B). Sixteen patients sustained injuries from falls (group C), and 2 had their hands crushed while using an industrial press machine (group D). In group A, most patients had a fracture line starting from the middle section of the radial side, accompanied by osteosclerotic changes in their lower hand. In group B, most patients had fracture morphologies similar to those of most patients in group A. The main morphology of fractures in group C was a transverse fracture at the base. Two patients in group D had minimal fragments at the tip of the hooks.

Conclusions: Our results question the theory that most hamate hook fractures in athletes are caused by direct force exerted on the palm because the fracture morphology was different from that of patients injured by acute trauma from direct force. Instead, these fractures likely stem from an indirect mechanism involving repetitive force generated by the tendons and muscles acting on the hook.

Keywords: baseball; diagnosis; fracture/dislocation; hamate; hook of hamate fracture; sports injury; wrist.

Figure 1.

A photograph of the palm of the lower hand (the hand closest to the grip end side when the player held a bat) of a baseball player. Note. The dot indicates the center of the hamate hook, confirmed under fluoroscopy (black arrow). The skin callus is visible on the radio-distal side (white arrow).

Figure 2.

Computed tomography images of group A: baseball players. Note. (a) Axial image of a pitcher who was hit by a batted ball. A basal transverse fracture was observed. (b) Axial image observed in 18 patients. The fracture line running between the middle section of the radial side and the base of the ulnar side, accompanied by osteosclerotic changes. (c) Axial images of 5 patients with incomplete fractures, depicting the line solely on the radial side in the axial section. (d) Sagittal images of 5 patients with incomplete fractures, illustrating the line solely on the proximal side in the sagittal section.

Figure 3.

Computed tomography images of group B (other athletes). Fracture morphology similar to that in most group A is observed. Note. (a) Axial image. (b) Sagittal image.

Figure 4.

Computed tomography axial images of group C: patients injured during a fall. Note. (a) Main fracture morphology, a transverse fracture at the base, was observed in 8 patients (9 hands). (b) Fractures lying between the ulnar side and the tip and middle part of the radial cortex (n = 3). (c) Comminuted fracture of the hamate hook and comminuted fracture including the hamate body observed in 3 patients and 1 patient, respectively (n = 3). (d) Comminuted fracture including the hamate body and hook (n = 1). (e) Fractures similar to those in most groups A and B (n = 1).

Figure 5.

Computed tomography images of group D. Note. (a) Minimal fragments observed at the tip of the hook on the axial image. (b) Patients in group D have concomitant dislocation fractures of the fourth and fifth carpometacarpal joints. The sagittal image shows the fourth carpometacarpal joint dislocation fracture.

Figure 6.

Computed tomography images of the corresponding author’s left hand gripping a rigid urethane foam stick, similar in diameter to a baseball bat (30 mm). Note. (a) Axial image sliced at level a’ (gray solid line in the sagittal image at the lower left of the figure). The urethane foam does not appear in this slice, and the palm is not caved by the stick. (b) Coronal image sliced at level b’ (gray dotted line in the sagittal image at the lower left of the figure). The arrow indicates the hamate hook, and the flexor tendons from the middle to little fingers are kinking by the hook.

Figure 7.

Schemas of the injury mechanism. Note. (a) Basal transverse fracture due to a direct force from the palm. (b) When the direct force from the palm is radially deviated, the fracture could occur on the ulnar side to the tip and middle part of the radial cortex. (c) Pressure from tightened finger flexors generates tension force at the radial side, initiating a fracture from the same side. (d) Contraction of the flexor digiti minimi induces tension at the proximal side, initiating a fracture from the same side.