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Sports Injury-Related Fingers and Thumb Deformity Due to Tendon or Ligament Rupture

Background: Hand injuries are very common in sports, such as skiing and ball sports. One of the major reasons causing hand and finger deformity is due to ligament and tendon injury. The aim of this study was to investigate if the high-resolution 3T magnetic resonance imaging (MRI) can demonstrate the complex anatomy of the fingers and thumb, especially the tendons and ligaments, and provide the accurate diagnosis of clinically important fingers and thumbs deformity due to ligamentous and tendinous injuries during sport activities. Methods: Sixteen fresh un-embalmed cadaveric hands were harvested from eight cadavers. A total of 20 healthy volunteers' hands and 44 patients with fingers or thumb deformity due to sports-related injuries were included in this study. All subjects had MR examination with T1-weighted images and proton density-weighted imaging with fat suppression (PD FS) in axial, coronal, and sagittal plane, respectively. Subsequently, all 16 cadaveric hands were sliced into 2-mm thick slab with a band saw (six in coronal plane, six in sagittal plane, and four in axial plane). The correlation of anatomic sections and the MRI characteristics of tendons of fingers and the ulnar collateral ligament (UCL) at the metacarpal phalangeal joint (MCPJ) of thumb between 20 healthy volunteers and 44 patients (confirmed by surgery) were analyzed. Results: The normal ligaments and tendons in 16 cadaveric hands and 20 volunteers' hands showed uniform low-signal intensity on all the sequences of the MRI. Among 44 patients with tendinous and ligamentous injuries in the fingers or thumb, 12 cases with UCL injury at MCPJ of the thumb (Stener lesion = 8 and non-Stener lesion = 4), 6 cases with the central slip injury, 12 cases with terminal tendon injury, and 14 cases with flexor digitorum profundus injury. The ligaments and tendons disruption manifested as increased signal intensity and poor definition, discontinuity, and heterogeneous signal intensity of the involved ligaments and tendons. Conclusions: Sports injury-related fingers and thumb deformity are relatively common. MRI is an accurate method for evaluation of the anatomy and pathologic conditions of the fingers and thumb. It is a useful tool for accurate diagnosis of the sports-related ligaments and tendons injuries in hand.

ulnar collateral ligament (UCL). Because this ligament is very small and difficult to see on routine radiography and computed tomography (CT), magnetic resonance imaging (MRI), a noninvasive method, can provide a high-resolution image of the soft tissue in multiple planes and visualize the detailed anatomic structures of the ligaments and tendons of the hands. [7][8][9][10][11][12][13][14][15] MRI, especially high-resolution 3T MRI is the preferred imaging modality in the diagnosis of the ligamentous and tendinous injuries in fingers and thumb. A comprehensive understanding of the detailed anatomy of the hands and the corresponding MRI features can aid in the diagnosis and treatment of sports-related injuries of ligaments and tendons.
This study was designed to outline the anatomy and MRI appearance of the normal ligaments and tendons in the hands including the UCL at the metacarpal phalangeal joint (MCPJ) of the thumb, the central slip, the terminal tendon, and the flexor digitorum profundus (FDP) using cadaveric specimens and normal volunteers, review the common sports-related fingers and thumb deformity due to ligamentous and tendinous injuries, discuss the mechanisms of injuries, and illustrate the characteristics of MRI using the surgically proven cases of ligamentous and tendinous injuries.

Ethical approval
The study was adhered to the Declaration of Helsinki and approved by the Institutional Review Board of Beijing Jishuitan Hospital. Written informed consent was obtained from all participants in this study.

Cadavers and subjects
The 16 cadaveric hands were harvested from eight cadavers that provided by the Institute of Basic Sciences, Chinese Academy of Medical Sciences. There were five men and three women with age range between 30 and 60 years at death with mean age of 46 years. All specimens included the distal radius and ulna, entire wrist and hand, which were kept at −42°C (Haier BioMedical, DW-40W100, Qing Dao, China) for the following experiments. All specimens were thaw at room temperature for 24 h before MRI was taken.
The normal MRI features of ligaments and tendons in the hands were analyzed on 80 fingers and 20 thumb from 20 healthy volunteers (10 men and 10 women, right hands: 5 men, 5 women; left hands: 5 men, 5 women; age range from 21 to 53 years with mean age of 32 years) in Beijing Jishuitan Hospital from March 2014 to June 2016 without history of ligamentous and tendinous injuries in hands.
Forty-four patients with ligaments or tendons injury who were confirmed by surgery between March 2013 and June 2016 were included and analyzed in this study. In these patients, 12 cases with UCL injury at the MCPJ of thumb, 6 cases with the central slip injury, 12 cases with terminal tendon injury, and 14 cases with FDP injury.
To be included in this study, the healthy volunteers must have no history of hand trauma, free of pain, with normal range of motion in hands. The patients who had UCL injury at the MCPJ must have the history of trauma and localized pain and swell in the ulnar aspect of the MCPJ. In addition to the specific traumatic history, pain and swell, the patients with injuries of extensor mechanisms must have troubles in extending the corresponding joint even the whole finger actively. The patients who had injuries of the FDP were unable to flex the distal interphalangeal joint (DIPJ) with the proximal interphalangeal joint (PIPJ) held in extension.

Magnetic resonance imaging
MR examination was performed with a 3T MRI unit (5680 DA Best, Philips Medical Systems, Netherlands) with a 16-channel hand and wrist receiver only coil (Philips Hand/Wrist 16 3T Tim coil). The healthy volunteers and patients were positioned prone with the examined arms placed above the head, and the hands were placed close to the isocenter of the gantry. The imaging planes for axial, coronal, and sagittal acquisitions must be prescribed with respect to the individual finger rather than the hand. An adjacent finger should be included within the field of view (FOV) to allow internal comparison. The cadaveric specimens were placed in the same way.
The fingers of all cadaveric specimens, volunteers, and patients underwent MRI with the following sequences: What was deserved to be mentioned was that the MRI of thumb was taken on axial, oblique coronal, and oblique sagittal planes with the specimen's thumb abduction and other parameters were same to the fingers.

Anatomic specimens preparation
After obtained MRI, all cadaveric specimens were immediately frozen in neutral position at −42°C and were kept at this temperature for at least 24 h. Those specimens were subsequently sliced into 2-mm thick slices (that corresponded to the section thickness of the MRI) with a stainless steel band saw (American Meat Equipment Corp, Montebello, USA). Fingers and thumbs in six specimens were sectioned along the coronal plane, six along the sagittal plane, and four along the axial plane. Each slice was digitally photographed (EOS 6D, Canon, Japan), and colored photographs were obtained for the anatomic correlation analysis with the corresponding MRI.

Magnetic resonance imaging: Anatomic comparison and analysis
All MRI were interpreted independently by two musculoskeletal radiologists who had 5-10 years of experience. Both radiologists were blinded to the clinical data, including the findings in the surgery reports.
Any discrepancy would be resolved by consensus with introducing an additional radiologist with more than 10 years of experience. Every UCL at the MCPJ of thumb, central slip, terminal tendon, and FDP of fingers was analyzed in the 16 cadaveric hands and 20 volunteers' hands. The appearance and signal intensity of normal ligaments and tendons on MRI in 16 cadaveric specimens were compared with the appearance derived from the inspection of the corresponding colored photographs obtained from anatomic slices. The MRI characteristics of normal ligaments and tendons in 20 healthy volunteers' hands were analyzed and confirmed. In the 44 patients, the corresponding injured structures would be referred to the normal anatomy and analyzed. In addition, the MRI characteristics and results were compared with the surgical results in a double-blind manner.

Gross anatomy and magnetic resonance imaging characteristics of the related ligaments and tendons
The normal ligaments and tendons of fingers and thumb in 16 cadaveric hands and 20 volunteers' hands showed hyposignal intensity on all the MR sequences [ Figures 1-6].

Ulnar collateral ligament at the metacarpal phalangeal joint of thumb
The UCL at the MCPJ originated from the ulnar aspect of the dorsal tubercle of the first metacarpal and inserted into the base of the proximal phalanx. The UCL was mainly stabilized by the adductor aponeurosis (AA), which lay superficial to the UCL and attached at the ulnar aspect of the proximal phalanx [ Figures 1 and 2]. With thumb abduction, the AA could be seen in every specimen on oblique coronal images of the thumb. The normal UCL showed as a thicker band than the AA with homogeneously hyposignal intensity. The average thickness is between 2.0 and 2.3 mm. The UCL and the AA could be best visualized in the oblique coronal plane on T1-weighed image (T1WI) and PD FS [ Figures 1 and 2].

Central slip
The central slip arose from the extensor digitorum communis and inserted on the base of the dorsum of the middle phalanx of fingers [ Figure 3]. The normal central slip demonstrated as a low signal line with a clear border on both T1WI and PD FS. The central slip could be best evaluated in axial and sagittal planes [ Figures 4 and 6]. With section thickness 2 mm, the central slip could be identified on at least two consecutive slices on sagittal images.

Terminal tendon
The terminal tendon was formed by the conjoint tendons at the level of the midportion of the middle phalanx and traveled from the distal side of the middle phalange to the dorsum of the base of the distal phalanx [ Figure 3]. The terminal tendon was similar to the central slip showing as a low signal line and could be best visualized in axial and sagittal planes on both T1WI and PD FS [ Figures 4 and 6].

Flexor digitorum profundus
The flexor tendons of the index, middle, ring, and the little fingers included the flexor digitorum superficialis (FDS) and FDP. At the level of MCPJ, the FDS split into two beams bypassing the FDP, while they remerged into a beam at the PIPJ and inserted on the midportion of the middle phalanx [ Figure 3]. The FDP passed superficially at the level of the proximal phalanx and inserted on the volar base of the distal phalanx. On both T1WI and PD FS, the flexor tendons showed as low-signal intensity, and they could be best seen on axial and sagittal planes [ Figures 4-6].

Sports-related injuries on magnetic resonance imaging Gamekeeper's thumb (Skier's thumb)/Stener lesion
The term Gamekeeper's thumb was applied to both acute and chronic injuries of the UCL at the MCPJ of thumb. [16] When the torn UCL retracted and lay superficial to the AA, this lesion was termed Stener lesion [ Figure 7]. [16] Among    Figure 7]. Among the Stener lesions, there were five cases with the torn ligament occurred at the insertion into the base of the proximal phalanx, three cases with the torn ligament occurred at the midportion of the ligament. The remaining four cases with non-Stener lesion showed the thickening and partial or total ruptured UCL with hyperintense on PD FS and the continuity of the AA. There were bone marrow edema at the base of the proximal phalanx and the surrounding soft-tissue edema in all 12 cases and among them; there were four cases with avulsion fracture at base of the proximal phalanx. This type lesion was best evaluated on coronal plane.

Boutonniere deformity
The boutonniere deformity referred to disruption of the central slip at the insertion into the base of the middle phalanx and leaded to the flexion of the PIPJ and extension at the DIPJ [ Figure 8]. There were six cases with the disruption of the central slip and showed the discontinuity of the normal

Jersey finger
The jersey finger referred to the injury of the distal FDP tendon at the base of the distal phalanx [ Figure 10]. It usually occurred when the finger was forcibly extended and it was commonly happened with an avulsion fracture of the volar aspect of the distal phalanx base. The best image to demonstrate this type of injury is on the sagittal PD FS images, when there is complete tear of the tendon, the discontinuity of the hypointense tendon along the volar, the extent of retraction, and the increased signal intensity could be best visualized [ Figure 10]. There were five patients with ring finger injury, five with little finger injury, three with the middle finger, and one with the index.

diScuSSion
Ligaments and tendons injury of the hand are relatively common in sports, [17] and some of the most common injuries   hypointense linear tendon with retraction. In the acute case, there was adjacent soft-tissue edema. The sagittal was the best plane to demonstrate this deformity [ Figure 8]. In these six patients, four people had injuries in the little finger, and two in the index finger.

Mallet finger
The mallet finger was caused by the rupture of the terminal tendon at its distal insertion on the dorsal aspect of the distal phalangeal base and resulted in the flexion of the DIPJ. This type injury was often accompanied by the avulsion fracture of the dorsal aspect of the distal phalanx base [ Figure 9]. Twelve cases with the rupture of the terminal tendon showed the discontinuity of the hypointense of the tendon, which could be evaluated on the sagittal PD FS [ Figure 9]. Among these 12 patients, five people had injuries in the ring finger, five in the little finger, and two in the middle finger. Among the 12 patients, one patient had the avulsion fracture of the dorsal aspect of the distal phalanx base in the little finger. resulting in the disability of the hands. Therefore, the accurate and early diagnosis is of vital significance. MRI, with its excellent soft-tissue resolution and multiplanar imaging capabilities, has become a preferred modality in evaluation of ligamentous and tendinous injuries in the hands and fingers. [18][19][20][21][22][23][24] The Gamekeeper's thumb/Skier's thumb including Stener lesion results from a directed force on an abducted thumb, most often related to the skiing injuries. [25,26] In the skiing sport, the skier grips his/her ski pole when sustains a fall with the thumb in abduction position may result in a valgus/radial stress on the thumb. [16] The radial side stress applied to the ulnar side of the distal thumb, across MCPJ, and caused the UCL injury. This type of lesion is also seen in football, basketball, and other contact-collision sports. [17] Spaeth et al. [10] described the Stener lesion as "yo-yo on a string" appearance with the string representing the AA, while the yo-yo representing the ruptured and retracted UCL. [27,28] In our study, we found that the disruption and displacement of the UCL were best seen on coronal plane, which confirmed what Cockenpot et al. stated. [26] In our study, among these Stener lesions, there were five cases with rupture of UCL at the insertion into the proximal phalanx, which confirmed the statement that in most cases of UCL injuries, most of them occurred at the distal insertion. [17] We assumed that the main reason was the insertion of the UCL which located in the base of the proximal phalanx increased the possibility of injury when the thumb was hyperabducted. In addition, a Stener lesion was an indication for surgical repair. [16] Boutonniere deformity results from the forced flexion of the PIPJ [ Figure 8], a blow to the dorsum of the middle phalanx or volar dislocation of the PIPJ or the middle phalanx. [27][28][29][30][31][32] Boutonniere deformity is most commonly seen in basketball and volleyball players. [32] Closed extension splinting is the initial treatment option for the boutonniere deformity, because splinting alone typically allows the central slip to heal. [16] Mallet finger results from the forced flexion of the extended DIPJ [ Figure 9]. [29,30,[33][34][35] This type of injury is typically observed in softball, basketball, baseball, volleyball, or in receivers in football. [16,17,36] Posner [37] ever stated that the mallet finger was the most common closed tendon injury seen in sports, while there were only 12 cases in the 44 patients with sports-related deformity in our study. This discrepancy might be due to the small sample size in our research. There were five patients had injuries in the ring finger, five in little finger, and two in the middle finger. Those findings basically confirmed what Cockenpot et al. said that the mallet finger occurred most in the middle, ring, and little fingers. [26] It is important to notice that the approximate percentage of articular surface involvement in cases with bony avulsion of the base of the distal phalanx, the gap between the fracture fragments, and palmar subluxation and dislocation of the distal phalanx in the radiographs of osseous injuries. If the imaging findings consist with a fracture of the base of the distal phalanx with a dorsal fragment that involves >30% of the articular surface, fragment diastasis >3 mm, or palmar subluxation of the distal phalanx with respect to the middle phalanx, surgical management is recommended. [16,33] Jersey finger is the result of the forced hypertension of the DIPJ while the DIPJ is actively flexed [ Figure 10]. [38,39] It is commonly seen in football, flag football, or rugby. [26] Some scholars have stated that this type deformity is most commonly seen in ring finger. [17,26,31] Hong found that the ring finger is involved in up to 75% of this type injury. [40] However, the involved ring finger accounts for 5/14 in our study. This discrepancy might be due to the small sample size in our research.
In conclusion, sports-related fingers and thumb deformity due to ligament and tendon injury is common, and the early treatment plays profound effect on the functional recovery and preventing permanent deformity. MRI has high resolution of soft tissue and can be used to visualize the ligaments and tendons in hand. It plays a significant role in detecting the injuries and assessing the extent and instructing the most appropriate treatment protocols.

Conflicts of interest
There are no conflicts of interest.