Ulnar Superficialis Slip Resection Versus Radial Superficialis Slip Resection: A Biomechanical Pilot Study

Abstract

Background:

There are numerous clinical scenarios during which the surgeon contemplates whether the radial or ulnar slip of the flexor digitorum superficialis (FDS) should be sacrificed. To date no study has assessed the point of failure of each one of the FDS slips in each digit, aiding the avid surgeon in deciding which slip to sacrifice.

Methods:

A total of 41 digits were assessed, each digit was dissected, and a specimen containing the denuded bone of the middle phalanx with the attachments of the ulnar and radial FDS slips was obtained. An Instron 4502 device was utilized to biomechanically assess the point of failure of each slip of each digit.

Results:

There was no statistical difference between ulnar and radial slip point of failure when compared across all digits and subjects. There was no statistical difference between male and female subject’s specimens. The point of failure was higher in the ulnar slips of the second and third digits, whereas the point of failure was higher in the radial slips of the fourth and fifth digits.

Conclusions:

Sacrifice of a FDS slip may cause loss of grip strength. In several clinical scenarios one may be faced with the dilemma which FDS slip to sacrifice. Our findings show this is not an arbitrary choice. Hand surgeons should keep our findings in mind when deciding which slip to sacrifice, in effort to preserve function and strength in the injured hand.

Introductory

The flexor tendons of the hands are comprised of 2 layers: The superficial layer includes the flexor digitorum superficialis (FDS), and the deep layer includes the flexor digitorum profundus (FDP). At the metacarpophalangeal joints (MCPJ), the FDS tendon splits into 2 slips with the FDP tendon running between them, forming the Camper’s chiasm. The 2 slips insert onto the middle phalanx. They primarily act to flex the proximal interphalangeal joint (PIPJ). ¹

The traditional classification of flexor tendon injuries involves 5 zones. Zone II is defined as the region from the distal palmar crease to the insertion of the FDS. This zone’s complex anatomy has been associated with poor outcomes following surgical repair and was coined “No man’s land.” ² One of the problems with repairing the tendons in this area is “overstuffing” of the sheath, which can impair tendon gliding. ³

Sacrifice of one of the slips of the FDS has been suggested as a possible solution for “overstuffing” or “crammed” flexor tendon repairs.^3-12 Sacrifice of one of the FDS slips can be considered as the treatment of choice when enlarged tendons are encountered following chronic synovitis (and thus finger triggering and stiffness) and trigger finger conditions including pediatric trigger finger.^13,14

It is traditionally accepted that sacrifice of the ulnar slip of the FDS is the procedure of choice when sacrifice of either slip of the FDS is considered.^3,4,7,8 It is unclear in the current literature whether there is a biomechanical superiority for choosing to sacrifice the ulnar slip over the radial slip. We performed a biomechanical assessment aimed at assessing whether there is a difference in the point of failure in either slip of the FDS in digits 2 through 5 in the human hand.

Materials and Methods

This study was approved by the local ethics committee.

Preparation

A total of 41 digits were attained from cadavers embalmed in formaldehyde. Due to religious reasons in our country, there are no fresh specimens. Age range of the specimens is of older patients. The digits were meticulously dissected, and a specimen containing the denuded bone of the middle phalanx with the attachments of the FDS preserved was obtained.

The combined FDS tendon was incised at myotendinous junction while avid precautions were taken to preserve the respectful tendon fibers comprising each slip of the FDS. Each slip was marked ulnar/radial. The specimens were kept in gauze laden with formaldehyde until biomechanical testing of the specimens was performed.

Measurements

An Instron 4502 (Instron Inc. Massachusetts) device was utilized to assess each specimen (Figure 1). The Instron 4502 is a tension testing device and is used to determine the material stiffness, that is, the material’s ability to resist deformation under tension.

Figure 1.

The Instron 4502 (Instron machine Inc. Massachusetts). Slip after being pulled by the Instron machine and reaching its breaking point and tearing from intra-substance.

Each specimen was mounted on the device and pull-out of each slip of the FDS was performed. The mid phalanx was connected to the upper clamp and one of the slips (either radial or ulnar, marked) was connected to the lower clamp of the Instron machine. The tear was observed from intra-substance of each slip.

The pullout speed was 1 millimeter/second. The calculated maximal load was measured in units of Newton (N). Following pullout of each slip the mounting clamps were locked once again and the other slip was pulled out.

The data collected by the Instron device was transformed to excel sheet format and later statistical analysis was performed via a SPSS module, version 25.

Results

Of the total 41 specimens assessed within the scope of this study 11 were from the second digit, 12 were from the third digit, 12 were from the fourth digit and 6 were from the fifth digit. The results were considered significant when the p-value was less than .05. Results are shown in Table 1.

Table 1.

Main Results.

Digit no.	Radial slip, max. load	Ulnar slip, max. load	Paired sample correlation	Significance
Second	207.2 ± 60.5	218.5 ± 39.2	.238	.568
Third	239.0 ± 61.9	275.6 ± 62.0	.535	.057
Fourth	183.1 ± 41.3	177.5 ± 56.1	.684	.642
Fifth	68.0 ± 26.0	54.5 ± 16.9	.518	.203

There was no statistical difference between ulnar and radial slip pull out strength when compared across all subjects. There was no statistical difference between male and female subjects’ specimens.

Discussion

This is a biomechanical study. The researchers used the Instron 4502 device, which is a tension testing device and is used to determine the material stiffness, that is, the material’s ability to resist deformation under tension. A material can be loaded and plot stress versus strain, therefore creating the material’s “Stress-Strain Curve.” The curve shows the elastic limit of the material (beyond this point, the material’s structure is irreversibly changed), the ultimate strength (maximum strength obtained by the material) and the breaking point (the material fractures).^1,15,16

The Stress-Strain curve of tendons is not linear. A tendon under tension will initially extend before elastic extension, and then extend until deformation. The curve typically consists of an initial non-linear toe region, a linear slope and then the failure region. The ultimate tensile stress is reached once the tendon is at its maximum value of stress.^1,15,16 The point of failure is of much interest.

Our results showed a tendency toward the ulnar slip of the FDS being the dominant slip in the 2^nd and 3^rd digits with higher loads to failure, whereas the ulnar slip in the 4^th and 5^th digits was found to be the non-dominant slip of the FDS with higher load to failure. The low number of specimens did not allow to draw further conclusions. We performed power analysis that showed that the number of specimens needed for statistical significance and power of at least 80% are 320, 46, 1164, and 42 for the second digit, third digit, fourth digit, and fifth digit, respectively.

Nonetheless, these findings represent a paradigm shift in the way we believe the slips of the FDS should be perceived. Our findings suggest that since the higher load to failure slip in the second and third digits is the ulnar FDS slip it should not be sacrificed^4,7,8 but rather the radial slip of the FDS should be sacrificed in these digits. Marcus et al ⁷ as well as Rubin et al ⁸ chose to sacrifice the ulnar slips of the FDS in 2 different clinical settings; trigger finger release in diabetics and chronic granulomatous tenosynovitis respectively, based on the assumption that the ulnar FDS slip is the less dominant one in all digits. Our findings challenge this belief: in the second and third digits the breaking point of the ulnar slips of the FDS was greater and hence we perceive the ulnar slip as the dominant one and the radial FDS slip should be sacrificed in these digits. Moreover, when conceptualizing the grip process and when considering the concepts of the mobile and stable arch, it is easily conceived that the ulnar slip is stronger than the radial one in the second and third digits in the human hand.

One of the limitations of our study was the quality of the specimens. The specimens were preserved in formaldehyde and might not allow for true and exact biomechanical data as fresh cadavers due to loss of flexibility of some of the tissues involved and specifically tendons. Although the insertion point to each digit could be different for each digit, it does not affect the results since the comparison was made for each digit between its ulnar and radial slip. Each person has obviously a different insertion size and tendon, and that’s why it’s important to gather as many specimens as possible to eliminate this difference. Our research is limited by the donations made to the pathological lab.

Most of our specimens came from a population of older aged individuals who knowingly donate their bodies for use by the medical community, and hence we believe the values attained here to be lower than those that would be found in younger individuals. Another limitation of the study is the small sample size. A larger sample size might have benefited more significant results, as reflected in our power analysis.

Although the cadavers were not fresh and from elderly donors, all tendons were processed and preserved in the same fashion and therefore comparable and hence we believe make our results valid. Meticulous attention was paid to keep all data accretion in line.

Others have reported a wide range of forces acting on flexor tendons depending on activity. The average load to failure of both slips in our study seems to be sufficient for active rehabilitation and most activities of daily living, exceeding physiologic loads typically encountered. ¹⁷ Each slip could independently provide sufficient FDS strength during active motion, thus allowing the surgeon to consider excising either of them, depending on individual clinical factors. However, in activities of higher demand, the breaking point might be limited. Vigouroux et al estimated finger muscle tendon tensions and pulley forces during specific sport-climbing grip techniques and found mean external grip force of 95.6 N in the crimp grip and 97 N in the slop grip. They didn’t examine each digit individually, but at least for the 5^th digit, according to our results, failure would occur at 54.5 N to 68 N. ¹⁸

When met with the need to sacrifice a slip of the FDS the avid hand surgeon should keep our findings in mind, and if necessary, can sacrifice the ulnar slip in the ulnar fourth and fifth digits while in the second and third digit the surgeon can choose to sacrifice the radial slip of FDS.

Our findings offer a new insight to management of injuries to the digit flexor mechanism and specifically Zone II injuries also designated “No man’s land,” where sacrifice of one FDS slips is one of the treatment options for avoidance of overstuffing after a tendon repair or treatment of chronic synovitis. ³ - ¹² Further studies are needed to establish our results and conclusions.