The Application of Sesamoid Position in Diagnosing Thumb Metacarpophalangeal Joint Dorsal Dislocation: A Retrospective Study

Abstract

Objectives

The position of sesamoid of thumb metacarpophalangeal (MCP) joint changed clearly when the joint was dislocated dorsally. However, the significance of sesamoid location in diagnosing joint dislocation was unclear. The present study aimed to explore the positional relationship between sesamoid bone and thumb metacarpophalangeal joint in normal and dorsal dislocation joints.

Methods

Between January 2018 and August 2023, we collected 60 isometric plain films from sixty outpatients and reviewed 56 anisometric plain films from twenty‐eight emergency patients with dorsal dislocation of thumb MCP joint at Tianjin Hospital, then took measurements on the hand X‐ray images. The sesamoid length on its longitudinal axis was defined as DP, the distance between the distal edge of sesamoid and thumb MCP joint was defined as DJ, and the ratio of DJ and DP was R. An independent‐samples t‐test and paired‐samples t‐test was utilized to analyze difference among data groups.

Results

The 60 isometric images were from 30 male and 30 female outpatients with normal bone structure in their hands, and the 56 anisometric images of the 28 emergency patients included both preoperative and postoperative materials. Among the outpatients, the actual distance between the distal edge of sesamoid and thumb MCP joint space (DJ) was 2.09 mm and 1.40 mm in males and females, respectively. The authentic average length of sesamoid (DP) was 4.46 mm in males and 4.22 mm in females. The average value of R (the ratio of DJ and DP) in males and females was 0.49 and 0.34, respectively. There were gender‐related statistical differences in DJ (p < 0.01) and R (p=0.01), but no statistical difference in DP (p > 0.05). For the 28 emergency patients, the mean value of R was –0.47 before joint reduction and 0.58 after joint reduction, with statistical difference between them (p < 0.01).

Conclusions

There was significant difference in the relative position between sesamoid and thumb MCP joint when joint dislocation and joint reduction. The distal edge of sesamoid beyond thumb MCP joint could be an evidence in diagnosing joint dorsal dislocation. The distal edge of sesamoid below thumb MCP joint could be an evidence of joint reduction.

In standard posterior‐anterior images, when the distal edge of the sesamoid beyond the first metacarpal head, indicating that joint dislocation with large possibility. When the distal edge of the sesamoid below the first metacarpal head, indicating that normal joint or it has been reduced.

Introduction

Sesamoid bone is the minimum bony structure of humans, which is embedded in the volar plate of fingers. The time between the onset of sesamoids of thumb metacarpophalangeal (MCP) joint and development corresponds to the physiological period of puberty in all individuals. ¹ Double sesamoids are frequently found in the first MCP joints. ² However, single sesamoid is seen in the other MCP joints and the thumb interphalangeal (IP) joint. ³ , ⁴ , ⁵ , ⁶ , ⁷ Few studies have observed sesamoids in proximal interphalangeal (PIP) joint of middle, ring and little fingers. ⁸ A study reported sesamoids were found in distal interphalangeal (DIP) joints. ⁴ Moreover, there are ethnic, gender and side differences in sesamoid distribution. African, Middle‐eastern and European ancestries are susceptible to sesamoids. ² Females are more frequently found to have sesamoids in MCP 2, MCP 5 and thumb IP joints than males. Moreover, sesamoids are more frequently found in bilateral or unilateral right side, compared to unilateral left side. ⁶

Sesamoid bone plays a major role in MCP joints, such as stabilizing joints, alleviating friction and improving grip. ² In 1966, Kleinberg et al. first proposed the definition of sesamoid index, which multiplied by the greatest diameter and its perpendicular diameter in the same sesamoid image. ⁹ Sesamoid index is used as a supplement in diagnosing acromegaly ⁹ and psoriasis. ¹⁰ There are several reports on sesamoid fracture ¹¹ , ¹² and sesamoid incarceration in joint dislocation patients. ¹³ , ¹⁴ Conservative treatment is sufficient for simple fractures, ¹⁵ while open surgery is needed for concurrent collateral ligament avulsion, volar plate rupture and sesamoid incarceration. ¹⁶ A unique study analyzed the relationship between sesamoid distribution and finger independence, and hypothesized that the hypermobile finger has higher frequency of sesamoids. ¹⁷

There are very few studies on the locational relationship between sesamoid and MCP joints. We found thumb MCP joint dorsal dislocation usually combined with migratory sesamoids in clinical practice. However, sesamoids as an important component of the joint, has never been applied in diagnosing joint dislocation. Therefore, this study aimed to explore: (i) sesamoid length and the distance between the distal edge of sesamoid and thumb MCP joint space; and (ii) the positional variations of sesamoid when joint dislocation and joint reduction. This is the first study to definitively elucidate the positional relationship between sesamoid bone and thumb MCP joint in both morbid and normal conditions.

Methods

Inclusion and Exclusion Criteria

The inclusion criteria included: (i) adults with normal isometric images; or (ii) patients with dorsal dislocation of thumb MCP joint; and (iii) standard posterior–anterior images. The exclusion criteria included: (i) adolescent; (ii) bony abnormity like acromegaly, psoriasis, rheumatoid arthritis, osteoarthritis, congenital deformity and hand fracture; (iii) other MCP joint dislocation except thumb; and (iv) volar dislocation of thumb MCP joint.

Study Design and Subjects

Between January 2018 and August 2023, we collected 60 isometric plain films from 60 outpatients and carried out radiological examination due to exclude abnormity of bony structure at Tianjin Hospital, Tianjin, China. We also reviewed 28 patients who were diagnosed with dorsal dislocation of the thumb MCP joint by physical and radiologic examinations from the emergency department, and acquired their preoperative and postoperative materials which consisted of the 56 anisometric hand X‐ray images.

As dorsal dislocation of thumb MCP joint is more common than volar dislocation, ¹⁸ the latter was excluded in this study. Moreover, as the radial sesamoid overlaps with the first metacarpal head in standard posterior–anterior images, therefore the ulnar sesamoid was measured in this study. All data were measured on the longitudinal axis of the first metacarpal in order to ensure accuracy and consistency.

Data Collection and Definitions

The data on age, gender and side of the patients were recorded. Since the primal images of the emergency patients were conducted at different institutions, we only collected these anisometric image materials. The isometric images of the outpatients and the anisometric images of the emergency patients were prepared. The length of the thumb sesamoid bone and its distance from the MCP joint space was measured (Figure 1). The dorsal cortex of the first metacarpal was regarded as a straight line, and its perpendicular line (line a) was drawn intersecting with the dorsal and the volar cortex of the first metacarpal at point A and point B, respectively. Thereafter, the perpendicular bisector line (line b) of the line segment (point A to point B) was made using a compass, which was regarded as the longitudinal axis of the first metacarpal. Finally, three horizontal lines, which were vertical to the longitudinal axis, were drawn through the proximal edge of sesamoid (line c), the distal edge of sesamoid (line d) and the distal edge of the first metacarpal head (line e) using a parallel ruler. Thus, three intersecting points were obtained, that is, point P, point D and point J. The distance of point D and point P (DP) was the longitudinal length of sesamoid. DJ (point D to point J) was the distance from the distal edge of sesamoid to the thumb MCP joint space. If line d was located distal to line e, the value of DJ was recorded as minus. The value of R was the ratio of DJ and DP, for comparing between isometric and anisometric imaging materials. For maintaining consistency, all data were measured on the longitudinal axis of the first metacarpal with a vernier caliper (precision = 0.02 mm). Each group of data was simultaneously measured by three researchers, and the mean was selected.

FIGURE 1.

The dorsal cortex of the first metacarpal was regarded as a straight line (the rough red line), and its perpendicular line (line a) was drawn intersecting with the cortex of the first metacarpal at point A and point B. The perpendicular bisector line (line b) of line AB was made as the longitudinal line of the first metacarpal. Three horizontal lines, which were vertical with line b were outlined through the proximal edge of sesamoid (line c), the distal edge of sesamoid (line d) and the distal edge of the first metacarpal head (line e), respectively. There were three intersecting points between the three horizontal lines and line b, i.e., point P, point D and point J. The distance between point P and point D was DP, the distance between point D and point J was DJ, and the ratio of DJ and DP was the value of R.

A Handy Method Was Designed for Checking Joint Condition

We designed a convenient method for verifying thumb MCP joint dislocation in order to simplify procedures at emergency and outpatient department. A square rule and standard posterior–anterior X‐ray image was sufficient. First, one of the square borders overlapped with the dorsal cortex of the first metacarpal. Thereafter, the other square border overlapped with the distal edge of the sesamoid. If the second square border beyond the distal edge of the first metacarpal head, indicating that the sesamoid position was abnormal, then the thumb MCP joint was dorsally dislocated (Figure 2). Conversely, if the second square border below the distal edge of the first metacarpal head, indicating that the joint was normal or had been reduced (Figure 3).

FIGURE 2.

We ensured that one of the square borders overlapped with the dorsal cortex of the first metacarpal and the other square border overlapped with the distal edge of the sesamoid, the horizontal square border beyond the distal edge of the first metacarpal head.

FIGURE 3.

The horizontal square border below the distal edge of the first metacarpal head.

Statistical Analysis

Differences in DJ, DP and R of the outpatients based on side and gender were analyzed using an independent‐samples t‐test. The values of R before and after joint reduction of each emergency patient were analyzed through paired‐samples t‐test by SPSS version 26.0 (IBM, Armonk, NY, USA). Statistical significance was set at p < 0.05, and the number was accurate to two decimal places.

Results

Basic Information of Patients

This study included 60 outpatients, with 30 males and 30 females, and the mean age was 40.3 years and 43.5 years, respectively. There were 10 right hands and 20 left hands in both male and female outpatients. Moreover, there were 28 thumb MCP joint dorsal dislocation patients (the mean age was 32.0 years) from the emergency department, including five females in total. Five of the emergency patients underwent open surgeries due to unsuccessful manual reduction, and the others underwent closed reduction and splint external fixation. No definite differences were found in DJ, DP and R on gender and side for the outpatients (p > 0.05). The difference of age between the outpatients and the emergency cases had statistical significance (p < 0.01).

Sesamoid Length and its Location in Different Genders and Different Joint Conditions

Among the outpatients, the authentic sesamoid length on its longitudinal axis in males (DP = 4.46 mm) was slightly larger than that in females (DP = 4.22 mm), but without statistical significance (p > 0.05). The actual distance between the distal edge of the sesamoid and thumb MCP joint space (DJ) in males and females was 2.09 mm and 1.40 mm. Sesamoids were closer to the MCP joint space in females compared to males, with evident statistical significance (p < 0.01). The average value of R in males and females was 0.49 and 0.34 with statistical difference in gender (p = 0.01). In the 28 emergency patients, the values of R had prominent difference (p < 0.01) between joint dislocation and joint reduction. Interestingly, all R values were negative before reduction while becoming positive after reduction, and the mean was −0.47 and 0.58, respectively.

Discussion

The actual size of sesamoid, the authentic distance between the sesamoid and thumb MCP joint space, and the positional variance of sesamoid after joint dislocation remain unknown. Therefore, we conducted this preliminary study and obtained some authentic data, such as the length of the sesamoid and its distance to thumb MCP joint space. Meanwhile, we found sesamoids moved distally when joint dorsal dislocation compare with normal joints.

The Length of Sesamoid and its Distance to Thumb MCP Joint Space

There are very few reports on the specific size of sesamoid. This study provided preliminary rough information by measuring isometric images, although it is not a multidimensional study. The average longitudinal length of sesamoids was 4.46 mm in males and 4.22 mm in females. Sesamoids of males were larger than that of females, but showed no significant gender‐based trend. This finding was in agreement with the reports of Kleinberg et al. ⁹ and Whitehouse et al. ¹⁰ The distance between the distal edge of the sesamoid and thumb MCP joint space in males and females was 2.09 mm and 1.40 mm. Sesamoids were significantly closer to thumb MCP joint space in females compared to males. This information had not been reported to date. Further studies will need to be carried out to demonstrate this phenomenon.

Sesamoid Position Changed Distinctly after Joint Dislocation

We performed comparative analysis of the positional relationship between sesamoid and thumb MCP joint based on the value of R, as there had no isometric images of the emergency patients. For the 28 patients, the distal edge of the sesamoid was beyond the thumb MCP joint space when joint dislocation, hence the values of R were all negative. In contrast, the distal edge of sesamoid was below the thumb MCP joint space after joint reduction, so positive values of R were obtained. These two groups of R values were statistically different, and the distal edge of sesamoid beyond the MCP joint space could be used in diagnosing thumb MCP joint dorsal dislocation. We did not draw a comparison between outpatients and emergency patients in the data for the following reasons. First, the age in outpatients and emergency patients was significantly different, the mean age of emergency patients was younger than outpatients (32.0 and 41.9 years, respectively). Second, materials in the two groups were not the same size. Finally, we performed self‐controlled study on the emergency patients in the conditions of joint dislocation and joint reduction.

Limitations and Strengths

The limitations of this study included its retrospective design, a small sample size, error caused by artificial measurement, and lack of multidimensional research. Regardless, this is the first study to elucidate the positional relationship between sesamoid bone and thumb MCP joint in both morbid and normal conditions, and provide actual information about sesamoid length and the distance between sesamoid and thumb MCP joint.

Conclusions

In conclusion, this study provided a rough length of the sesamoid on its longitudinal axis, and the distance between the sesamoid and the thumb MCP joint space. The positional relationship between sesamoid and thumb MCP joints showed significant differences when joint dislocation and joint reduction. The distal edge of sesamoid beyond thumb MCP joint could be evidence in diagnosing joint dorsal dislocation. The distal edge of sesamoid below thumb MCP joint could be an evidence of joint reduction.

Authors Contribution

All authors had full access to the data in the study and take responsibility for the integrity of the data and the accuracy of the data analysis. Conceptualization: Haihua Zhan and Jianhua Xu; methodology: Jianhua Xu; investigation: Bo Zhang, Shuming Cao and Dake Zhu; formal analysis: Li Han; resources, Li Han; Writing—original draft, Jianhua Xu; Writing—review and editing, Zhonggang Yin; visualization, Jianhua Xu; supervision: Ketong Gong.

Conflict of Interest Statement

The authors declare that they have no conflict of interest.

Authorship Declaration

All authors listed meet the authorship criteria according to the latest guidelines of the International Committee of Medical Journal Editors and all authors are in agreement with the manuscript.