Finger flexor tendon forces are a complex function of finger joint motions and fingertip forces.

In vivo tendon forces provide a view inside the musculoskeletal system revealing muscle function and potential injury etiologies. The studies presented here measured the in vivo tendon force of the flexor digitorum superficialis of the long finger during open carpal tunnel release surgery in ten adult patients. Forces were measured during passive movement of the finger, isometric pinch, and dynamic tapping of the finger. The tendon forces during passive movement of the finger were the largest with the finger fully extended. During isometric pinch, tendon force was linearly related to fingertip force, and was on average 3.3 times larger than the fingertip force. During dynamic activities, however, the relationship between tip and tendon force was nonlinear and often remained elevated when the finger was moving but with no applied force. Tendon forces were the highest with the isometric finger pinch. In conclusion, tendon force is a completed function of both fingertip load and motion of the joints that the tendons cross. A comparison of these results with others published in the literature indicated that rehabilitation processes need to incorporate a systems approach rather than rely on one specific physiologic relationship to minimize finger flexor tendon forces.     

In vivo flexor tendon forces increase with finger and wrist flexion during active finger flexion and extension.

The effects of different hand motions and positions used during early protected motion rehabilitation on tendon forces are not well understood. The goal of this study was to determine in vivo forces in human flexor digitorum profundus (FDP) and flexor digitorum superficialis (FDS) tendons of the index finger during active unresisted finger flexion and extension. During open carpal tunnel surgery (n = 12), flexor tendon forces were acquired with buckle force transducers, and finger positions were recorded on video while subjects actively flexed and extended the fingers at two different wrist angles. Mean in vivo FDP tendon forces varied between 1.3N +/- 0.9 N and 4.0 N +/- 2.9 N while mean FDS tendon forces ranged from 1.3N +/- 0.5 N to 8.5 N +/- 10.7 N. FDP force increased with active finger flexion at both wrist angles of 0 degrees or 30 degrees flexion. FDS force increased with finger flexion when the wrist was in 30 degrees flexion, but was unchanged when the wrist was in 0 degrees of flexion. Tendon forces were similar regardless of whether the fingers were moving in the flexion or extension direction. Active finger flexion and extension with the wrist at 0 degrees and 30 degrees flexion may be used during early rehabilitation protocols with limited risk of repair rupture. This risk can be further decreased for a FDS tendon repair by reducing wrist flexion angle.     

Flexor tendon forces: in vivo measurements.

S-shaped force transducers were developed for measurement of the forces along intact tendons. After calibration, the transducers were applied to the flexor pollicis longus and flexor digitorum superficialis and profundus tendons of the index finger in five patients operated on for treatment of carpal tunnel syndrome. The tendon forces generated during passive and active motion of the wrist and fingers were recorded. For pinch function, the amount of the applied load was measured with a special pinch meter. Tendon forces in the range of 0.1 to 0.6 kgf were measured during passive mobilization of the wrist. Tendon forces up to 0.9 kgf were present during passive mobilization of the fingers. Tendon forces up to 3.5 kgf were present during active unresisted finger motion. Tendon forces up to 12.0 kgf were recorded during tip pinch, with a mean applied pinch force of 3.5 kgf. These results have potential application in determining the amount of force that a tendon repair would have to resist during passive as well as active postsurgical mobilizations.     

Quantifying catch‐and‐release: The extensor tendon force needed to overcome the catching flexors in trigger fingers

The extensor tendon forces required to overcome the catching flexors in trigger fingers are unknown. A biomechanical model with moment equilibrium equations and method of least squares was developed for estimating the tendon force at triggering in trigger fingers. Trigger fingers that exhibited significant catching and sudden release during finger extension were tested. A customized “pulling tester” was used to pull the finger from flexion to extension and provide synchronic measurement of the pulling force. The displacement of the tested finger was measured by a motion capture system. This preliminary study presents kinematic and kinetic data at triggering of 10 trigger fingers. The distal and proximal interphalangeal (PIP) joints presented sudden release while the metacarpophalangeal (MCP) joint started extension in the early phase of finger extension. The tendon tension of flexor digitorum profundus was greater than that of flexor digitorum superficialis (FDS) in six fingers, and less than that of FDS in three fingers. The tension of two flexor tendons was almost equal in one finger. At the PIP and MCP joints, 1.54 times the force of flexors was needed for the extensors to overcome the catching flexors in trigger fingers. This biomechanical model provides clinicians with a clearer idea of the tendon force at triggering. The quantitative results may help in the understanding of movement characteristics of trigger fingers. These findings are useful to better understand the etiology and nature of trigger finger development, and thus aid in further development of better assessments and treatments related to this. © 2013 Orthopaedic Research Society Published by Wiley Periodicals, Inc. J Orthop Res 31:1130–1135, 2013     

Differential splintage for flexor tendon rehabilitation: an experimental study of its effect on finger flexion strength

We conducted laboratory tests to investigate the possibility of partly de-powering flexor digitorum profundus with a view of reducing flexion force during active flexor tendon rehabilitation. We constructed a splint and applied tapes to the proximal segments of fingers to test the hypothesis that holding three fingers more extended than the other finger would reduce the flexion strength of the more flexed finger. The splint allowed the metacarpophalangeal joint of the more flexed finger to be held in three positions of increasing flexion (15 degrees , 30 degrees , and 45 degrees ) compared to the remaining three fingers. We have called this 'differential splintage'. Healthy volunteers were tested for maximum active flexion strength at the different flexion angles. 'Differential splintage' of up to 45 degrees resulted in mean decreased flexion strength of 28% in the index finger and 35% to 38% in the middle, ring and little fingers. The results suggest that "differential splintage" of a finger after flexor tendon repair may be useful in reducing tension across the repair during a program of active tendon rehabilitation and we feel that it has potential to reduce the incidence of repair rupture before healing is complete.     

Releasing the A3 pulley and leaving flexor superficialis intact increases pinch force following the Zancolli lasso procedures to prevent claw deformity in the intrinsic palsied finger.

Objective estimates of fingertip force magnitude following surgery to prevent digital metacarpophalangeal (MCP) hyperextension (clawing) in cases of paralysis of the hand's intrinsic muscles will assist clinicians in setting realistic expectations for post-operative pinch strength. We used a cadaveric/optimization approach to predict and confirm the maximal biomechanically possible fingertip force in the intrinsic palsied hand before and after two popular tendon transfer methods to the volar plate of the MCP joint. Both surgeries were also evaluated after release of the A3 pulley-a modification predicted by our published computer model of the forefinger to increase fingertip force magnitude. We predicted maximal static fingertip force by mounting eight fresh cadaveric hands on a frame, placing their forefinger in a functional posture (neutral abduction, 45 degrees of flexion at the MCP and proximal interphalangeal joints, and 10 degrees at the distal interphalangeal joint) and pinning the distal phalanx to a 3D dynamometer. We pulled on individual tendons with tensions up to 25% of maximal isometric force of their associated muscle and measured fingertip force and torque output. Using these measurements, we predicted the optimal combination of tendon tensions that maximized palmar force (analogous to pinch force, directed perpendicularly from the midpoint of the distal phalanx, and in the plane of finger flexion-extension) for four cases: (i) the non-paretic case (all muscles available), (ii) intrinsic palsied hand (no intrinsic muscles functioning), (iii) transfer of flexor superficialis tendon to the volar plate of the MCP (Zancolli lasso) in the intrinsic palsied hand, and (iv) leaving flexor superficialis intact and transferring a tendon of comparable strength to the volar plate of the MCP in the intrinsic palsied hand. Lastly, we applied these optimal combinations of tension to the cadaveric tendons and measured fingertip output. With the A3 pulley intact, the maximal palmar force in cases (ii)-(iv) averaged 48 +/- 23% SD (non-paretic = 100%; case (iv) (61 +/- 25%) > cases (ii) and (iii) (43 +/- 23% and 39 +/- 19%, respectively), p < 0.05). Releasing the A3 pulley significantly increased the average palmar force in cases (ii)-(iv) (73 +/- 42%, p < 0.05), with no significant differences among them. Thus, releasing the A3 pulley may improve palmar force magnitude when it is necessary to transfer the digit's own flexor superficialis tendon to the volar plate of the MCP to prevent clawing in the intrinsic palsied hand.     

Cyclical testing of zone II flexor tendon repairs

Kessler, Strickland, or modified Becker repairs, all augmented with a running circumferential epitenon suture, were performed for simulated zone II flexor tendon lacerations in the index, long, and ring fingers of 12 fresh-frozen cadaveric specimens. Each hand was tested with a tensiometer built for curvilinear testing of human flexor tendons in an intact hand. Each tendon was cycled 100 times, then examined for gapping before testing to failure. Maximum load to failure, including tendon load and pinch force, was recorded for each tendon. We propose that combining the advantages of cyclical testing and a curvilinear model is the most effective way of testing flexor tendon repairs capable of undergoing an early active motion protocol. None of the repaired tendons failed during the cyclic portion of testing. The average gapping after cycling for the 3 suture techniques was 0.12 +/- 0.35 mm for the Kessler technique, 0. 00 +/- 0.00 mm for the Strickland technique, and 0.19 +/- 0.26 mm for the modified Becker technique. The maximum tendon loads to failure were 33.8 +/- 6.8 N for the Kessler technique, 30.4 +/- 5.64 N for the Strickland technique, and 76.3 +/- 9.02 N for the modified Becker technique. There was a statistically significant difference between the modified Becker repair and the other 2 repairs for maximum tendon load and pinch force to failure. The results of this study show that all 3 tendon repair techniques can withstand forces reported with passive motion, but only the modified Becker repair allows sufficient strength above those forces that are estimated for active motion during tendon healing.     

Tensile load on the flexor digitorum profundus tendon during palmar and lateral blocking exercises: Influence on blocking force and distal interphalangeal joint flexion angle

Study DesignThis is a basic science research.IntroductionIsolating excursion of the flexor digitorum profundus (FDP) in zones I and II is common practice in the current management after flexor tendon repair. During this procedure, the proximal interphalangeal joint is sometimes fully extended with unmeasured external forces at the middle phalanx when the distal interphalangeal joint is actively flexed.Purpose of the StudyThe purpose of the study was to investigate the incremental effect of external force with palmar blocking versus lateral blocking and increased angles of flexion on internal tendon forces at the repair site for a safer application of force by the treating therapist.MethodsEight human cadaveric fingers were studied. To simulate palmar or lateral finger blocking, a compression force of blocking was applied from 5N (510 grams) to 25N (2,550 grams) on the skin surface of the palmar or the lateral aspect of each of these middle phalanges in 5N increments. The tensile load on the FDP tendon during distal interphalangeal joint flexion from 0° to 60° was measured in 10° increments.ResultsDuring palmar blocking, the tensile load was significantly increased with increases in palmar blocking force. However, no significant increase in the tensile load on the FDP tendon was observed at any lateral blocking.DiscussionLateral blocking exercise can be performed with less tensile force on the FDP tendon when performing blocking exercise after flexor tendon injury repair.ConclusionsThis study supports the concept that lateral blocking with incremental joint angles allows a safer application of force for the healing tendon.     

Dorsal-enhanced sutures improve tension resistance of tendon repair

Thirty-six fresh pig flexor tendons were repaired using either the modified Kessler method or the Tang method. Nine tendons from each group were tested in an Instron tensile testing machine with the tendons passing 90 degrees around a pulley. The other nine tendons from each group were pulled linearly by the testing machine. The 2 mm gap formation force of the tendons repaired with the modified Kessler and Tang methods and pulled at 90 degrees were 64%+/-5% and 79%+/-9% respectively of those forces recorded during linear testing. The ultimate strengths of tendons repaired by the modified Kessler and Tang methods and pulled at 90 degrees were 76%+/-6% and 81%+/-8% respectively of the forces measured during linear testing. The percentage gap formation and ultimate strength of the Tang method was significantly higher than that of the modified Kessler suture when the tendons were pulled around a pulley. This demonstrates that the Tang suture, with its main components in the dorsal part of the repaired tendon, has greater tension resistance capacity than conventional tendon sutures which are placed in the middle of the tendon. This study suggests that dorsally-enhanced multiple tendon sutures are better placed to sustain the tension generated during active finger flexion.     

Tendon and pulleys at the metacarpophalangeal joint of a finger.

In fresh frozen traumatically amputated forearms with a constant tension of one kilogram on the flexor profundus tendon and the interphalangeal joints fixed in full extension by a Kirschner wire, the excursion of the tendon at the metacarpophalangeal joint and the force at the finger tip were correlated with different angles of flexion of the joint, first with the finger intact and then after varying amounts of advancement of the metacarpophalangeal joint pulley system. Pulley advancement increased the tendon excursion required to flex this joint and thus the mechanical advantage at this joint, but only when the joint was partly flexed. The extra excursion required at the metacarpophalangeal joint would be expected to weaken the interphalangeal joints at full flexion. Advancement also permitted ulnar-radial displacement of the tendon at the level of the metacarpophalangeal joint and hence could accentuate ulnar or radial drift. Pulley advancement is not recommended.     

The mechanical properties of human flexor tendons in relation to artificial tendons

The continuation of an unacceptable failure rate with tendon repair or grafting procedures, largely due to adhesions, suggested that an artificial flexor tendon could be an attractive alternative. A literature search found no published data of the mechanical properties of fresh human finger flexor tendons, so a study of the strength and extensibility of 153 tendons was carried out. The bone insertion strength of twenty middle finger tendons was also examined. The results showed that an artificial tendon should have a strength of approximately 1500N, and that it should extend 13% at that load, an elongation of 26mm for a tendon 200mm long. The insertion strength was less than a half of the tendon strength. This data will allow an artificial flexor tendon to be designed with sufficient strength and the correct elastic properties to allow its function to integrate reliably with natural tendons in adjacent fingers.     

A comparison of four repair techniques for Camper's chiasma flexor digitorum superficialis lacerations: tested in an in vitro model

The relative strengths of 4 methods for repair of the flexor digitorum superficialis tendon were examined in 14 fresh-frozen cadaver hands (40 tendons). All tendons underwent sharp zone II transection at Camper's chiasma. All transections were repaired with 4.0 Ethibond (Ethicon Inc, Sommerville, NJ) using modified Becker, modified Kessler, horizontal mattress, or simple sutures. Flexion of the repaired digit at a constant excursion rate was rendered up to tendon rupture. The modified Becker technique withstood breaking forces (57.9 N) significantly greater than the other techniques examined. Forces up to 34 N have been measured in vivo during unresisted active finger motion. Thus, the modified Becker technique appears to provide adequate strength for early active flexor digitorum superficialis motion.     

Avulsion of the flexor digitorum profundus: anatomic and biomechanical considerations

Avulsion of the profundus insertion occurs most commonly in the ring finger. The exact reason for this predilection is unknown. Clinical observation of patients with this injury reveals that a common finding is that the ring fingertip is usually more prominent or "longer" than any other fingertip during grip. A laboratory investigation shows that during grip the ring fingertip becomes 5 mm more prominent than any other digit in 90% of subjects and that it absorbs more force than any other finger during pull-away testing. These factors contribute to the susceptibility of the ring finger to the profundus avulsion injury.     

Effect of elbow position on canine flexor digitorum profundus tendon tension.

Tendon injury in the finger remains a clinical challenge to hand surgeons. A canine model is commonly used to study biological effects of tendon injuries and their treatment. There is an important anatomical difference between human and canine anatomy that may be overlooked, however, namely that most of the flexor digitorum profundus (FDP) muscle in dogs takes its origin from the medial epicondyle of the humerus, whereas in humans this muscle arises purely from the forearm. Therefore, elbow position can affect the tension of this muscle in dogs, while having no effect in humans. The purpose of this study was to measure the effect of elbow position on tendon tension in the canine digit in vitro. Elbow position had a significant effect on tendon tension. Digit motion with the elbow fully flexed resulted in significantly higher tendon tension compared to digit motion with the elbow flexed 90 degrees or fully extended, regardless of digit or wrist position (p<0.05). The tension with the elbow flexed 90 degrees was also significantly higher than with the elbow fully extended (p<0.05). The maximum tendon tension with the elbow fully flexed was more than eight times larger than that of the fully extended elbow (p<0.05). We conclude that, in the canine model, elbow position is an important parameter that affects the passive tension applied to the flexor digitorum profundus, and, by implication, to any repair of that tendon. Dog flexor tendon rehabilitation protocols should therefore specify elbow position, in addition to wrist and digit position.     

A comparison of four repair techniques for Champer's chiasma flexor digitorium superficialis lacerations: Tested in an in vitro model

The relative strengths of 4 methods for repair of the flexor digitorum superficialis tendon were examined in 14 fresh-frozen cadaver hands (40 tendons). All tendons underwent sharp zone II transection at Camper's chiasma. All transections were repaired with 4.0 Ethibond (Ethicon Inc, Sommerville, NJ) using modified Becker, modified Kessler, horizontal mattress, or simple sutures. Flexion of the repaired digit at a constant excursion rate was rendered up to tendon rupture. The modified Becker technique withstood breaking forces (57.9 N) significantly greater than the other techniques examined. Forces up to 34 N have been measured in vivo during unresisted active finger motion. Thus, the modified Becker technique appears to provide adequate strength for early active flexor digitorum superficialis motion. (J Hand Surg 2000;25A:1122-1126.     

The influence of wrist position on the minimum force required for active movement of the interphalangeal joints

Active and passive muscle tension is discussed in relation to finger flexor and extensor tendons. Minimising active tension required to produce finger movement is seen as an important part of post-operative finger mobilisation following flexor tendon repair in which active movement is used. It is argued that "minimal active tension" in the flexors is equal to, or just exceeds, the passive tension in the extensors. A method of measuring passive tension in finger tendons has been described. In 24 volunteers, it has been used to determine that if the metacarpo-phalangeal joints are held flexed, there is least "minimal active tension" in the flexor tendons when the wrist is splinted in extension.     

Rupture of the flexor digitorum superficialis: occurrence after tendon repair of an adjacent digit

A patient had a ruptured flexor digitorum sublimis tendon of the long finger in the region of decussation. The injury occurred several weeks after repair of a zone 2 flexor digitorum sublimis and flexor digitorum profundus tendon of the index finger.     

Effects of static fingertip loading on carpal tunnel pressure

The purpose of this study was to explore the relationship between carpal tunnel pressure and fingertip force during a simple pressing task. Carpal tunnel pressure was measured in 15 healthy volunteers by means of a saline-filled catheter inserted percutaneously into the carpal tunnel of the nondominant hand. The subjects pressed on a load cell with the tip of the index finger and with 0, 6, 9, and 12 N of force. The task was repeated in 10 wrist postures: neutral; 10 and 20° of ulnar deviation; 10° of radial deviation; and 15, 30, and 45° of both flexion and extension. Fingertip loading significantly increased carpal tunnel pressure for all wrist angles (p = 0.0001). Post hoc analyses identified significant increases (p <0.05) in carpal tunnel pressure between unloaded (0 N) and all loaded conditions, as well as between the 6 and 12 N load conditions. This study demonstrates that the process whereby fingertip loading elevates carpal tunnel pressure is independent of wrist posture and that relatively small fingertip loads have a large effect on carpal tunnel pressure. It also reveals the response characteristics of carpal tunnel pressure to fingertip loading, which is one step in understanding the relationship between sustained grip and pinch activities and the aggravation or development of median neuropathy at the wrist.     

Failure of flexor pollicis longus repair caused by anomalous flexor pollicis longus to index flexor digitorum profundus interconnections: a case report

We describe a case of an early rupture of a repaired flexor pollicis longus tendon in a young woman. The cause of failure was an anomalous tendinous band that connected the tendon of the flexor pollicis longus to the tendon of the flexor digitorum profundus of the index finger. Forceful flexion of the unrestricted index finger applied a tensile force that was transmitted through the anomalous band to the repaired site and resulted in repair failure.     

Repetitive differential finger motion increases shear strain between the flexor tendon and subsynovial connective tissue

Non‐inflammatory fibrosis and thickening of the subsynovial connective tissue (SSCT) are characteristic in carpal tunnel syndrome (CTS) patients. These pathological changes have been linked to repetitive hand tasks that create shear forces between the flexor tendons and SSCT. We measured the relative motion of the flexor digitorum superficialis tendon and SSCT during two repetitive finger tasks using color Doppler ultrasound. Twelve participants performed flexion?extension cycles for 30 min with the long finger alone (differential movement) and with all four fingers together (concurrent movement). Shear strain index (SSI, a relative measure of excursion in flexion and extension) and maximum velocity ratio (MVR, the ratio of SSCT versus tendon during flexion and extension) were used to represent shear. A linear effect of exertion time was significant and corresponded with larger tendon shear in differential motion. The flexion SSI increased 20.4% from the first to the 30th minute, while MVR decreased 8.9% in flexion and 8.7% in extension. No significant changes were found during concurrent motion. These results suggest that exposure to repetitive differential finger tasks may increase the risk of shear injury in the carpal tunnel. © 2013 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 31:1533–1539, 2013.