Ulnar coronoid process fractures are relatively uncommon injuries occurring in 2% to 10% of patients with elbow dislocations.17 Regan and Morrey described three types of coronoid fractures based on fragment size. Type I fractures involve a fracture of the tip, Type II fractures involve approximately 50% of the process, and Type III fractures involve greater than 50% of the process.19 As the fragment size increases, so does the incidence of an associated elbow dislocation and resultant joint instability.19 Consequently, it is recommended to fix all three types of fractures associated with elbow instability.20
Despite this clinical knowledge, there is a paucity of anatomic studies describing the three structures inserting on the ulnar coronoid process: the anterior bundle of the medial ulnar collateral ligament (MUCL), the anterior elbow capsule, and the brachialis muscle.3 There are several articles regarding the position and role of the anterior bundle of the MUCL.1,12,14,15 Of the different coronoid fractures, only Type III fractures include the MUCL insertion on the coronoid fragment.
We sought to better understand the specific location of the anterior elbow capsule and brachialis muscle on the ulnar coronoid process to better explain why instability also can be associated with some Type I and Type II coronoid fractures.
MATERIALS AND METHODS
Ten fresh-frozen cadaveric upper extremities, from 10 different cadavers, that were amputated at the midhumerus were obtained and prepared for fixation. Two were discarded because there was evidence of previous trauma. The remaining eight specimens had no evidence of surgery, trauma, or degenerative arthrosis. The age and gender of the cadavers were unknown. The specimens were dissected via a combined medial and lateral approach to the elbow taking care to preserve the coronoid process and its three associated soft tissue attachments: the anterior joint capsule, the brachialis insertion, and the anterior bundle of the MUCL. Medially, the dissection was centered along the medial intermuscular septum. The flexor and pronator origin was reflected to expose the MUCL and medial aspect of the coronoid process. Laterally, the interval between the triceps and brachioradialis was used proximally. The extensor origin was reflected from the lateral epicondyle, and the lateral capsule was opened in line with the midaxis of the radiocapitellar joint. The radial neck was osteotomized and the radial head was removed. The brachialis, medial collateral ligament (MCL), and capsule were transected leaving their ulnar attachments intact. A sagittal saw was used to remove a segment of the ulna including the coronoid process and its soft tissue insertions. Two saw cuts were made perpendicular to the long axis of the ulna, one through the midportion of the articular surface and one distal to the brachialis insertion, leaving a 5 to 6-cm bony segment.
We used the histologic techniques described by Torpey et al.24 The specimens were dehydrated with sequentially higher concentrations of alcohol, defatted with acetone, and dehydrated a second time in a pure alcohol solution. They then were infiltrated with methylmethacrylate (MMA) and a plasticizer before being embedded in MMA. Once the specimens were dry, they were cut longitudinally in the sagittal plane with a band saw, producing 3-mm thick sections. The blade thickness was 0.5 mm. The width of the coronoid processes determined the number of slices per specimen. Each specimen yielded two or three slices (Table 1). We stained the specimens with toluidine blue.
After staining, the sections were placed under a dissecting microscope and photographed (Fig 1). The photographs were digitized and measured using Sigmascan Pro software (SPSS, Inc, Chicago, IL). We measured the distance along the arc of the coronoid from the tip to the proximal aspect of the capsular and brachialis insertions. We also measured the height of the coronoid process relative to the volar cortex of the ulna. All specimens were measured once by one observer (CH). The averages and standard deviations (SD) represent the differences between slices from each specimen.
We performed a power analysis which determined that we would need approximately 200 specimens to achieve normative data and 20 specimens to obtain mean data. However, because of time and cost constraints, we were limited to 10 specimens. We used a Student's t test to compare capsular and brachialis insertion locations and coronoid height with those reported by Cage et al.3 We presumed a level of statistical significance of p < 0.05.
Our results showed a very proximal insertion of the capsule (Table 1). Given this location, virtually all Type I coronoid fractures, based on the classification system of (Morrey), should involve the anterior elbow capsule.19 The average tip to capsule distance was 2.36 ± 0.39 mm, and the average tip to brachialis distance was 10.13 ± 1.6 mm. The average coronoid height was 16.98 ± 2.5 mm.
Persistent elbow instability after a posterior elbow dislocation, particularly when associated with large coronoid fractures, is a difficult and well-described clinical occurrence.2,4-11,16,18,22,23 Two long-term studies have detected recurrent instability rates of 15% and 35%, respectively.10,13 Ring et al stated that elbow fracture dislocations with coronoid and radial head fractures are particularly prone to complications.21 Although there are some reports on how large coronoid fractures can lead to instability, little is written regarding the consequences of small coronoid fractures and whether they play a role in post-traumatic elbow instability.8,9,17,19-22 Terada et al described three patients in whom the reduction and fixation of small coronoid fractures restored anterior capsular integrity and stability.23 These patients had elbow dislocations with massive soft tissue disruption and proximal radial fractures, which remained unstable despite repair of the medial and lateral collateral ligaments and realignment of the radial fractures.23
Our study has several limitations. As earlier stated, our power analysis indicated we would need approximately 200 specimens to achieve normative data and 20 specimens to obtain mean data. For practical reasons, we were limited to 10 specimens but presumed these would provide at least new preliminary data. We were unable to evaluate the insertion of the anterior bundle of the MUCL because of our section plane. We selected the plane because it seemed to best show the capsule and brachialis. It would have been ideal to have software to provide three-dimensional or multiplanar depictions of the capsule and brachialis insertions.
Based on dissections of 20 specimens, Cage et al found the average distances from the tip of the coronoid to the proximal capsule and brachialis were 6.4 ± 2.7 mm (range, 0-10.4 mm) and 11 ± 2.6 mm (range, 4.1-15 mm), respectively.3 One specimen had capsular insertions at the coronoid tip, and two specimens had insertions in the Regan and Morrey Type I region.3 Our brachialis and coronoid tip height findings were similar to those of Cage et al. We did observe a more proximal location of the capsule insertions well within the Regan and Morrey Type I region (Table 2).3,19 Our findings implied most, if not all, coronoid process fractures would involve the capsular insertion.
Additional anatomic studies with sections in a different plane would provide direct histologic evidence regarding the insertion of the anterior bundle of the MCL. Our study potentially explains why instability can be associated with certain Type I coronoid process fractures.
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