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Intramuscular Tendon of the Adductor Pollicis and Underlying Capsule of the Metacarpophalangeal Joint: An Anatomical Study With Possible Implications for the Stener Lesion
Department of Orthopaedic and Spinal Surgery, Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University, Tokyo, JapanDepartment of Clinical Anatomy, Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University, Tokyo, Japan
Corresponding author: Akimoto Nimura, MD, PhD, Department of Functional Joint Anatomy, Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University, 1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8519, Japan.
Department of Orthopaedic and Spinal Surgery, Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University, Tokyo, JapanDepartment of Clinical Anatomy, Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University, Tokyo, Japan
To identify the layered relationship anatomically between the musculotendinous structures of the adductor pollicis, the ulnar collateral ligament, and the capsule of the metacarpophalangeal joint in terms of understanding the pathomechanism of a Stener lesion.
Methods
We macroscopically analyzed 37 cadaveric thumbs to identify the intramuscular tendon of the adductor pollicis and bony attachments of the joint capsule including the ulnar collateral ligament. In addition, we histologically analyzed 3 thumbs and made a 3-dimensional image of 3 other thumbs, using micro-computed tomography.
Results
The adductor pollicis has 3 components of an intramuscular tendon (dorsal, palmar, and distal), which connect to form a lambda shape. The dorsal part inserts into the joint capsule dorsal to the ulnar sesamoid. The palmar part inserts into the ulnar sesamoid. The distal part inserts into the lateral tubercle of the proximal phalanx. The thickened and cord-like part of the joint capsule, which has generally been referred to as the proper ulnar collateral ligament, has a distinct bony attachment on the proximal slope of the lateral tubercle of the proximal phalanx separate from the adductor pollicis insertion.
Conclusions
Of the 3 components of the intramuscular tendon of the adductor pollicis muscle, the dorsal part inserted into not only the aponeurosis but also the joint capsule.
Clinical relevance
The results of the current study suggest the anatomic basis for a possible pathomechanism of the Stener lesion.
Rupture of the ulnar collateral ligament (UCL) of the thumb metacarpophalangeal (MCP) joint is one of the most common ligamentous injuries of the hand. Failure to treat the injury may lead to pain and instability of MCP joint. In 1962, Stener
reported that in most thumb UCL injuries, the UCL was ruptured distally and was displaced proximally, with the aponeurosis of the adductor pollicis (AD) interposed between the torn end of the ligament and its attachment to the proximal phalanx. Stener concluded that this could prevent healing of the injured ligament. Many surgeons consider the Stener lesion to require operative treatment to avoid residual symptoms from this injury.
In the cadaveric dissections modeling this lesion, Stener
reported that after cutting the UCL, a maximum radial deviation force could not produce the Stener lesion. Based on these observations, the occurrence of the Stener lesion in vivo might be related to the combined effects of the connection between the UCL and the AD musculotendinous structure and contraction of the AD. Previous anatomic research on the ulnar side of the thumb MCP joint has been limited to the UCL itself
However, the layered relationship among musculotendinous structures of the AD, the UCL, and the MCP joint capsule, which is the deepest structure, has not been clarified.
The aim of the current study was to identify the layered relationship anatomically between the musculotendinous structures of the AD, the UCL, and the capsule of the ulnar side of MCP joint based on macroscopic, histological, and bone morphological methods.
Materials and Methods
We used a total of 44 thumbs (23 right and 21 left) from 31 Japanese cadavers (12 males and 19 females; average age, 83.3 years) in this study as a sample of convenience. We fixed all cadavers in 8% formalin and preserved them in 30% ethanol. The thumbs were amputated at the first carpometacarpal joint, including the attached muscles, and the skin and subcutaneous tissues were removed. One specimen with severe arthritic changes in the MCP joint was excluded. A total of 37 thumbs were used for macroscopic observations; 3 were for histological analysis and the remaining 3 were for observing bone morphology. First, we observed the outer appearance of the AD and removed the flexor pollicis longus tendon and overlying sheath. To reveal the intramuscular tendon of the AD, we removed the muscular parts. To understand the spatial layout of the intramuscular tendons, we sectioned the tendon just proximal to the insertion to the aponeurosis and the ulnar sesamoid. To reveal insertions of the intramuscular tendons of the AD, we detached the tendons from proximal to distal while preserving the MCP joint capsule. In addition, we sectioned the joint capsule just ulnar to the extensor pollicis longus tendon and detached the joint capsule from dorsal to palmar to identify the capsular attachment on the metacarpal and proximal phalanx. Of the 37 thumbs, we used 29 to measure the dimensions of the capsular attachment using a caliper, the other 8 thumbs were used for a preliminary study examining only the intramuscular tendons of the AD.
We analyzed 3 thumbs histologically. We removed the ulnar, palmar part of the MCP joint, parallel to the ulnar sesamoid and the lateral tubercle of the proximal phalanx, including the connection between the aponeurosis and the AD tendon, en bloc using a diamond band pathology saw (EXAKT 312, EXAKT Advanced Technologies GmbH, Norderstedt, Germany). We decalcified the specimen for 1 week in a solution containing aluminum chloride, hydrochloric acid, and formic acid, as described by Plank and Rychlo.
After decalcification, we sectioned the blocks in the oblique coronal plane at the center of the ulnar sesamoid and the dorsal part of the ulnar sesamoid, and dehydrated the blocks with a graded ethanol series. After dehydration, we embedded the tissue blocks in paraffin and serially sectioned them (5 μm thickness) parallel to the sectioned plane. We stained the sections with Masson trichrome.
To observe bone morphology, before dissection, we made 3-dimensional images of the last 3 thumbs using a micro-computed tomography (CT) scanner (inspeXio SMX-100CT, Shimadzu, Kyoto, Japan) with application software (VGStudio Max 2.0, Heidelberg, Germany). After imaging the 3-dimensional CT image to confirm the actual morphology of the bones without dissection artifact, we chemically removed the soft tissues of these 3 thumbs using a 0.8% sodium hydroxide solution (Wako Pure Chemical Industries, Osaka, Japan).
Results
Identification of intramuscular tendon of AD and its insertions on MCP joint
Before removing the muscular parts of the AD, we inspected the ulnar side of the MCP joint, the AD, and its insertion on the lateral tubercle of the proximal phalanx. The AD could be separated into transverse (ADT) and oblique (ADO) parts (Fig. 1A). We noted that there were 3 folded tendons, including one located distally (blue arrowhead in Fig. 1B), dorsally (red arrowhead in Fig. 1B, C), and palmarly (white arrowhead in Fig. 1C), which were connected to each other. To clarify the spatial layout of the 3 parts of the intramuscular tendon, we sectioned the tendon just proximal to the insertion on the ulnar sesamoid. The sectioned edge showed that the 3 parts of the intramuscular tendon connected to form a lambda shape (black dotted lines in Fig. 1C).
Figure 1Identification of intramuscular tendon of AD muscle. The ulnar and palmar aspect of a right thumb is shown. A Adductor pollicis muscle inserted into the bony prominence on the ulnar and volar part of the proximal phalanx (PP) base, which is called the lateral tubercle (asterisk). The adductor pollicis muscle was approximately divided into the transverse part (ADT) and oblique part (ADO). B The flexor pollicis longus tendon (FPL) and overlying sheath were removed. In addition, to reveal intramuscular tendons of AD, the muscular part was removed. Three folded tendons, which included the distal part (blue arrowhead), dorsal part (red arrowhead), and palmar part (white arrowhead in C), were identified to connect with each other. C To understand the spatial layout of 3 parts of the intramuscular tendon, we sectioned the tendon proximal to the insertion on the ulnar sesamoid. The sectioned edges of the tendon showed the lambdoid shape indicated as black dotted lines. Dors, dorsal; FPB, flexor pollicis brevis muscle; MC, metacarpal; PP, proximal phalanx; Prox, proximal.
To reveal the insertions of the 3 parts of the intramuscular tendon of the AD, we detached the tendons from the capsule of the MCP joint, ulnar sesamoid, and proximal phalanx. The dorsal part of the intramuscular tendon of the AD was connected to the AD aponeurosis (Fig. 2A) and the MCP joint capsule dorsal to the ulnar sesamoid (black arrows in Fig. 2B, C). The palmar part of the intramuscular tendon inserted into the ulnar sesamoid (section mark in Fig. 2C, D). The distal part of the intramuscular tendon inserted into the proximal-palmar prominence of the proximal phalanx, which has generally been called the lateral tubercle (asterisk in Fig. 2D).
Figure 2Insertions of intramuscular tendons of the AD. To reveal insertions of the 3 parts of intramuscular tendons of AD, we detached tendons from the capsule of the MCP joint, ulnar sesamoid, and proximal phalanx. The ulnar aspect of the MCP joint of the right thumb is shown in C. A The dorsal part of the intramuscular tendon of AD (red arrowhead) is connected to the aponeurosis (AP). B The AP was sectioned at the ulnar side of the extensor pollicis longus tendon (EPL) and reflected palmar. Note the AP and the proximodorsal part of the intramuscular tendon attached to the capsule dorsal to the ulnar sesamoid indicated with black arrows. C Dorsal and palmar parts of the intramuscular tendon were detached from the joint capsule and the ulnar sesamoid (section mark), respectively, and reflected together (AD). D The AD including the distal part of the intramuscular tendon (blue arrowhead) was detached from the lateral tubercle (asterisk) of the proximal phalanx. Similar marks represent insertions of the AD and corresponding parts of the AD. Dist, distal; EPB, extensor pollicis brevis tendon; MC, metacarpal; Palm, palmar; PP, proximal phalanx.
Capsular attachments of MCP joint capsule on metacarpal and proximal phalanx in relation to insertions of AD
In association with the AD insertions, to identify the attachments of the MCP joint capsule, we detached the joint capsule from the metacarpal and proximal phalanx from dorsal to palmar. After the AD was removed, the MCP joint capsule could be separated from the superficial tendons and aponeurosis (Fig. 3A). The joint capsule was sectioned at the ulnar edge of the extensor pollicis longus tendon and reflected palmarly. The joint capsule was widely attached to the metacarpal, palmar to the dorsal cartilage and proximal to the dorsal tubercle (open stars in Fig. 3B). The joint capsule that attached to this area of the metacarpal was thick and cord-like (white arrows in Fig. 3C, D). In addition, the thick and cord-like part of the joint capsule was attached mainly on the proximal slope of the lateral tubercle of the proximal phalanx. Except for the 2 attachments of the joint capsule described previously, the joint capsule continued to the periosteum, and thus the margin of the capsular attachment was unclear. Figure 4 and Table 1 show the average width and length of the attachment with the joint capsule and the insertion of the AD tendon.
Figure 3Capsular attachments on the metacarpal and proximal phalanx. To reveal attachments of the capsule of the thumb MCP joint, the joint capsule was detached from dorsal to palmar. A After removal of the AD, the ulnar aspect of the MCP joint of the right thumb is shown. Insertions of the AD on the ulnar sesamoid and the lateral tubercle of the proximal phalanx (PP) are indicated as a section mark and asterisk, respectively. The joint capsule was spared from superficial tendons and the aponeurosis. B The joint capsule was sectioned parallel to the long axis of the thumb from the dorsal edge of the MCP joint. The joint capsule was widely attached palmar to the dorsal cartilage and proximal to the dorsal tubercle of the metacarpal (MC) (open stars). C The capsule was detached in a more palmar direction. The thickened part of the joint capsule, which was attached on the open star area and is generally referred to as the ulnar collateral ligament (white arrows), was mainly attached on the proximal slope of the lateral tubercle (open circles). D The joint capsule was expanded in a palmar direction. The thickened part of the joint capsule was close to the ulnar sesamoid (US) and between the open star and open circle (white arrows). Similar marks represent attachments of the capsule and corresponding parts of the capsule. Dors, dorsal; FPB, flexor pollicis brevis muscle; MC, metacarpal; Prox, proximal; RS, radial sesamoid.
Figure 4Measurements of capsular attachment on the metacarpal and proximal phalanx and AD insertion. Black dotted area indicates the AD insertion on the lateral tubercle (Table 1). White dotted lines indicate the attachment of the joint capsule on the metacarpal (MC) and proximal phalanx (PP). Dist, distal; Dors, dorsal.
To understand the layered relationship between the intramuscular tendon of the AD and the joint capsule including the UCL, we made histological sections at the level of the ulnar sesamoid and lateral tubercle of the proximal phalanx and the area in which the dorsal part of the intramuscular tendon was connected with the MCP joint capsule dorsal to the ulnar sesamoid (Fig. 5A). At the level of the ulnar sesamoid and lateral tubercle, the AD and the joint capsule, including the ulnar collateral ligament, were joined; yet they were still distinguishable from each other and each had a separate attachment with fibrous cartilage on the proximal edge of the proximal phalanx (Fig. 5B). At the area dorsal to the ulnar sesamoid, the intramuscular tendon of AD inserted into and mingled with the joint capsule (Fig. 5C).
Figure 5Histological analysis of the insertion of the AD tendon. A Locations of the section are indicated as blue lines on the bone scheme of the thumb parallel to the AD tendon. B At the level of ulnar sesamoid (US), the AD and joint capsule including the UCL are adjoined. The joint capsule attached proximal to the AD insertion (asterisk) on the lateral tubercle of the proximal phalanx (PP) with a border, indicated as an open circle. C Dorsal to the US, the intramuscular tendon of the AD inserted into and mingled with the joint capsule (arrows). (Masson-trichrome stain; scale bars = 5 mm.) Dist, distal; Dors, dorsal; MC, metacarpal; Palm, palmar.
Confirmation of bony configurations of AD insertions and capsular attachment without dissection artifact
To eliminate dissection artifact caused by scraping fibrous cartilage from the tendinous insertions and capsular attachments, we confirmed the bony anatomy of the ulnar side of the MCP joint using 3-dimensional images of micro-CT and chemically removing soft tissues (Fig. 6). At the ulnar side of the proximal phalanx, a prominence (asterisk surrounded by white arrows in Fig. 6), which has generally been called the lateral tubercle, could be identified at the proximal and palmar base of the proximal phalanx. In addition, proximal to the lateral tubercle, the slope of the prominence (open circle in Fig. 6A–D) could be identified. At the ulnar side of the first metacarpal, a depression (open star in Fig. 6C, D) could be observed palmar to the dorsal cartilage and distal to the dorsal tubercle. These findings were consistent with the micro-CT 3-dimensional images and the specimens after chemical soft tissue removal.
Figure 6Bony morphology of the insertion of the AD tendon and the attachment of the joint capsule. To eliminated bias by detaching tendons and joint capsule using a scalpel, before dissection, cadaveric thumbs were examined directly using micro-CT (A, C) and soft tissues were chemically removed (B, D) to assess correlations with the micro-CT. The palmar (A, B) and ulnar sides (C, D), respectively, of the thumb MCP joint are shown. At the area proximal to the lateral tubercle (asterisk surrounded by arrows), which corresponds to the insertion of the distal part of the AD tendon, the different slope could be identified; it corresponds to the attachment of the MCP joint capsule including the ulnar collateral ligament (open circle). Dist, distal; MC, metacarpal; PP, proximal phalanx; US, ulnar sesamoid; Palm, palmar; Rad, radial.
described the transverse head as being directly inserted into the proximal phalanx, and the oblique head as inserting into the ulnar sesamoid. By removing muscular components and focusing on the tendinous parts of rotator cuffs, Mochizuki et al
identified that the supraspinatus footprint on the greater tuberosity was much smaller than previously believed, and the infraspinatus occupied a substantial area. In the current study, by removing muscular components in the same way, we clarified that the AD has 3 tendinous parts (distal, dorsal, and palmar), and they continued to be lambda shaped. We consider the previously described transverse and oblique heads of the AD to correspond approximately to the distal and palmar parts of the AD tendon, respectively, identified in the current study.
In addition, we identified the dorsal tendon as the third part of the intramuscular tendon of the AD. Henle
first described the muscle that originated from the first metacarpal and inserted into the aponeurosis of the AD as the pollical palmar interosseous muscle, whereas the palmar interosseous series in humans had been thought to consist of 3 muscles. Some articles supported that description and considered the dorsoradial slip of the AD to be the pollical palmar interosseous muscle; however, the presence of the dorsoradial slip varied in those reports.
These previously described slips of the AD, which have been considered identical to the pollical palmar interosseous muscle, could be assumed to be the dorsal part of the intramuscular tendon of the AD in the current study, and we consistently found it in all specimens. In the previous studies, these features were observed from the viewpoint of the muscular part. In the current study, by means of specifically analyzing the intramuscular tendons, we consistently found the dorsal part of the AD, and it was connected to other parts of intramuscular tendons. In addition, previous reports described the pollical palmar interosseous muscle to have an insertion into the aponeurosis of the AD, which was referred to as the “wing tendon.”
Based on the results of the current study, we have shown that the dorsal part of the intramuscular tendon inserts into not only the aponeurosis, but also the capsule of the MCP joint.
Previous studies of the anatomy of the thumb UCL focused on the location of the UCL. Bean et al
reported that the center of the phalangeal insertion of the proper UCL was 3 mm from the joint. However, the joint capsule has not been adequately described in anatomic studies of the ulnar side of the MCP joint, and hence the layered relationship between the UCL and the joint capsule remains unclear. In the shoulder and elbow joint, some authors described the bony attachment of the joint capsule as more complex than originally thought, which has some relevance for injuries to those structures.
Joint capsule attachment to the extensor carpi radialis brevis origin: an anatomical study with possible implications regarding the etiology of lateral epicondylitis.
Joint capsule attachment to the coronoid process of the ulna: an anatomic study with implications regarding the type 1 fractures of the coronoid process of the O’Driscoll classification.
Gamekeeper thumb: diagnosis of ulnar collateral ligament injury using magnetic resonance imaging, magnetic resonance arthrography and stress radiography.
assumed UCL to represent a capsular ligament like the medial collateral ligament of the knee. In the current study, we identified the joint capsule as the deepest layer of the MCP joint underlying the AD, and not based on a specific ligament such as the UCL. We showed that there was a thickened and cord-like part between the ulnar-side depression of the metacarpal and the proximal slope of the lateral tubercle of the proximal phalanx, which was embedded in the continuous sheet of the joint capsule. This thickened part of the joint capsule was assumed to be the proper part of the UCL contained in the capsule.
As for clinical relevance, we were able to provide some clues as to the nature of the UCL injury of the thumb MCP joint. First, based on the findings of the current study, we speculate that the Stener lesion results when, in response to a radially deviating stress applied to the thumb MCP joint, the AD eccentrically contracts and the dorsal part of the intramuscular tendon tears off the UCL proximal to the attachment on the proximal phalanx.
Second, regarding avulsion fractures associated with an UCL injury of the thumb MCP joint, some authors reported cases that included 2 fracture fragments.
In those cases, the larger fragment, lying adjacent to the base of the proximal phalanx, is unassociated with the UCL. The second, smaller fragment represents the actual torn end of the UCL (referred to as a “2-fleck sign”).
showed that the fragment originated from an avulsion of the AD tendon. Based on the current study, the distal part of the intramuscular tendon of the AD and the joint capsule, including the UCL, have distinct separate bony attachments: the lateral tubercle and proximal slope. In other words, the distal part of the AD and the UCL are not attached to the same area. That could be the reason for the 2-fleck sign described by Thirkannad and Wolff.
Because this study was purely anatomic and limited to uninjured specimens, we cannot prove the pathomechanism of the Stener lesion, and so our explanation remains speculative. To provide proof, additional biomechanical studies or imaging of clinical cases are needed based on the anatomic findings of the current study.
Acknowledgments
This study was partly supported by a grant from the JA Kyosai Research Institute (Agricultural Cooperative Insurance Research Institute).
References
Stener B.
Displacement of the ruptured ulnar collateral ligament of the metacarpophalangeal joint of the thumb.
Joint capsule attachment to the extensor carpi radialis brevis origin: an anatomical study with possible implications regarding the etiology of lateral epicondylitis.
Joint capsule attachment to the coronoid process of the ulna: an anatomic study with implications regarding the type 1 fractures of the coronoid process of the O’Driscoll classification.
Gamekeeper thumb: diagnosis of ulnar collateral ligament injury using magnetic resonance imaging, magnetic resonance arthrography and stress radiography.
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