Importance of proximal A2 and A4 pulleys to maintaining kinematics in the hand: a biomechanical study

Principles for Achieving Predictable Outcomes in Flexor Tendon Repair

Flexor tendon injuries require surgical repair. Early repair is optimal, but staged repair may be indicated for delayed presentations. Zone II flexor tendon injuries are the most difficult to achieve acceptable outcomes and require special attention for appropriate repair. Surgical techniques to repair flexor tendons have evolved over the past several decades and principles include core strand repair using at least a 4 strand technique, epitendinous suture to add strength and gliding properties, and pulley venting. Early postoperative active range of motion within the first 3 to 5 days of surgery is essential for optimizing outcomes.

What is new in flexor tendon pulleys and the gaps between them in triphalangeal fingers of the hand?

The flexor tendon pulleys in the fingers of the hand are fibrous structures of variable size, shape, and thickness that cover the synovial sheath of these tendons. Despite their clinical relevance, their arrangement and configuration in each of the triphalangeal fingers have been little studied and with small sample sizes. 192 triphalangeal fingers belonging to 48 fresh body donors' hands were dissected. Multivariate analysis was carried out. Twenty-five cases (52%) were left hands, and 26 of the 48 hands belonged to female donors (54.2%). The results were analyzed by fingers for each of the 5 annular pulleys, the 3 cruciform pulleys and the gaps between them. In addition, the most and least frequent configurations of the pulleys in each of the fingers were studied, observing that the classic pattern with all the pulleys appeared only in 3 fingers (1.56%), while the most frequent pattern was A1-A2-C1-A3-A4, which was seen in 35 fingers (18.22%). CONCLUSIONS: The flexor pulleys in the triphalangeal fingers of the hand have shown enormous variability in arrangement and shape, and also rarely appear all in the same finger. This peculiar anatomical arrangement can help the different professionals who perform their clinical work in this region.

Identifying Palmar Skin Surface Landmark for Locating A2 Pulley during Cadaveric Dissection of the Hand

The A2 and A4 pulleys are fibro-osseous structures that support the flexor tendon function. Injury to these pulleys can result in bowstringing and limited tendon excursion. Thus, having an understanding of the skin surface landmark of the A2 pulley is crucial to safeguard it during hand surgery.

Methods

We performed cadaveric dissection of 62 hands. For 248 fingers, the measurement of distance A, which is half the distance between the palmar digital crease and proximal interphalangeal crease reflected in the palm, and distance B, which is the distance between the A2 pulley's starting point and the palmar digital crease, were taken by a caliber. Statistical analysis was performed using the paired sample t test to determine whether there was a significant difference between distances A and B.

Results

Our study revealed that there was no significant difference (p>0.05) between the measured starting point of the A2 pulley and its proposed surface landmark for the index, middle, and small fingers. Conversely, the ring finger showed a statistically significant difference of 1 mm more proximal.

Conclusions

By measuring the distance between the palmar digital crease and proximal interphalangeal crease and reflecting it proximally in the palms, one can anticipate the location of the A2 pulley's starting point for each digit, except for the ring finger. The ring finger's starting point is 1 mm more proximal than the other digits. Knowing the starting point of the A2 pulley will help hand surgeons limit incisions and avoid accidental injury during hand surgery.

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 DESIGN

This is a basic science research.

INTRODUCTION

Isolating 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 STUDY

The 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.

METHODS

Eight 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.

RESULTS

During 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.

DISCUSSION

Lateral blocking exercise can be performed with less tensile force on the FDP tendon when performing blocking exercise after flexor tendon injury repair.

CONCLUSIONS

This study supports the concept that lateral blocking with incremental joint angles allows a safer application of force for the healing tendon.

The Impact of Dividing the Flexor Tendon Pulleys on Tendon Excursion and Work of Flexion in a Cadaveric Model

PURPOSE

The A2 and A4 pulleys of the flexor tendon system have traditionally been considered critical components of efficient digital flexion. This dogma has recently been challenged. Using fresh human cadaveric hands and a model to measure force and excursion, we sought to clarify the clinical importance of releasing different pulleys.

METHODS

Combinations of A1, A2, and A4 pulleys were released on the index, middle, ring, and little fingers of fresh, cadaveric hands. The excursion was measured as the distance the tendon was pulled by the motor to achieve palm touchdown. The force applied by the motor was constant (25 N); work was derived from the product of force and excursion (distance). The change in excursion and work needed to achieve palm touchdown before and after pulley release was measured. Excursion varies among digits and specimens at baseline; therefore, the percentage change from the intact state was used to compare groups. We compared A2 versus A1, A4 versus A1, A4 versus A2, A1 + A2 versus A2, and A1 + A4 versus A4.

RESULTS

Isolated A2 or A4 release had the greatest individual impact on the excursion (4.77% ± 1.52% and 3.88% ± 1.93%, respectively). When A1 was released with A2 (9.90% ± 2.52%), the additional impact on the excursion was significant; however, when A1 was released with A4 (2.63% ± 2.81%), the impact was marginal. No clinically or statistically significant change in the work of flexion was detected.

CONCLUSIONS

A1 release was clinically significant when added to A2 release but not when added to A4 release. Sacrifice of the A2 and A4 pulleys resulted in a statistically significant, but clinically negligible, difference in flexor tendon excursion. These data suggest that the A1 pulley should be preserved when other proximal pulley components are likely to be compromised. These data also add further support to the concept that the A2 pulley or the A4 pulley can be released as needed for optimal tenorrhaphy.

CLINICAL RELEVANCE

During flexor tendon repair, the length of contiguous pulley release may have more impact on final tendon excursion than which specific pulleys are released.

Comprehending Optimality of Finger Flexor Tendon Pulley System Using Computational Analysis

Existing prosthetic/orthotic designs are rarely based on kinetostatics of a biological finger, especially its tendon-pulley system (TPS). Whether a biological TPS is optimal for use as a reference, say for design purposes, and if so, in what sense, is also relatively unknown. We expect an optimal TPS to yield a high range of flexion while operating with lower tendon tension, bowstringing, and pulley stresses. To gain insight into the TPS designs, we present a parametric study which is then used to determine optimal TPS configurations for the flexor mechanism. A compliant, flexure-based computational model is developed and simulated using the pseudo-rigid body method, with various combinations of pulley/tendon attachment point locations, pulley heights, and widths. Results suggest that three distinct types of TPS configurations corresponding to a single stiff pulley, two stiff pulleys, or one stiff and one flexible-inextensible pulley per phalange can be optimal. For a TPS configuration similar to a biological one, the distal pulleys on the proximal and intermediate phalanges need to be like flexible-inextensible string loops that effectively model the behavior of joint and cruciate pulleys. We reckon that a biological flexor TPS may have evolved to maximize flexion range with minimum possible actuation tension, bowstringing, and pulley stress. Our findings may be useful in not only developing efficient hand devices but also in improving TPS reconstruction surgery procedures.

Is releasing pulleys during flexor tendon repair "part and parcel"? Narrative review of the current evidence

BACKGROUND

The complex hand flexors pulleys system is essential in achieving efficient flexor tendons' function. Previous cadaveric studies demonstrated that A2 and A4 are the crucial pulleys in maintaining normal digits biomechanics. Realistically, the preservation of A2 and A4 pulleys during repairing flexor tendon laceration in zones one and two can be extremely challenging. We review the current published evidence in this article to answer the question of whether releasing the pulleys cause bowstringing or affects clinical outcomes.

METHODS

Literature search of the available databases.

RESULTS

There was no published comparative evidence. Retrospective case series have reported that no clinical bowstringing was noted after releasing flexor pulleys during flexor tendon repairs. Outcomes have been reported according to Tang and or Strickland criteria to assess range of motion (ROM). No functional hand scores or patients' satisfactions scores have been reported.

CONCLUSION

Releasing flexor pulleys during tendon repair to allow access or prevent impingement of the repaired tendon does not seem to cause bowstringing or affect outcome based on the limited available evidence. Future research is needed.

LEVEL OF EVIDENCE

Level 4.

Use of Thermoplastic Rings Following Venting of Flexor Tendon Pulleys: A Biomechanical Analysis

PURPOSE

Normal digital flexion relies on flexor tendon pulleys to convert linear muscular force to angular digital motion. However, there is a growing trend to vent them partially during flexor tendon repair. The objective of this study was to examine the effects of a thermoplastic ring, acting as an external pulley, on flexor tendon biomechanics and finger range of motion (ROM) after pulley venting.

METHODS

We tested 15 cadaveric digits using an in vitro active finger motion simulator. We measured loads induced by flexor digitorum superficialis (FDS) and flexor digitorum profundus (FDP) as well as joint ROM with sequential sectioning of the A2, A3, and A4 pulleys compared with an intact pulley condition. At each stage, external thermoplastic pulley rings were applied snugly over the proximal and middle phalanges to recreate A2 and A4 function, respectively.

RESULTS

After complete venting of the A2, A3, and A4 pulleys, proximal interphalangeal joint ROM significantly decreased by 13.4° ± 2.7° and distal interphalangeal joint ROM decreased by 15.8° ± 2.1°. Application of external rings over the proximal and middle phalanx resulted in a residual ROM decrease of 8.3° ± 1.9° at the proximal interphalangeal joint and 7.9° ± 2.1° at the distal interphalangeal joint, nearly restoring ROM. Similarly, complete pulley venting resulted in reduced FDS load by 37% and FDP load by 50% compared with intact pulleys. After application of external rings, loads were restored almost to normal, with a 9% reduction for FDS load and 9% reduction for FDP load compared with intact pulleys.

CONCLUSIONS

The application of thermoplastic rings acting as external pulleys is an effective, noninvasive, and reproducible approach to restore flexor tendon biomechanics and digit ROM after pulley venting.

CLINICAL RELEVANCE

Thermoplastic rings may be a useful therapeutic adjunct in restoring joint ROM and flexor tendon loads after surgical venting of the pulleys.

Finger Flexor Pulley Injuries in Rock Climbers

Finger flexor pulley system injuries are the most common overuse injury in rock climbers. These injuries occur rarely outside of rock climbing, owing to the sport's unique biomechanical demands on the finger. As rock climbing continues to grow and earn recognition as a mainstream sport, an understanding of how to diagnose and treat these injuries also has become important. Our purpose is to describe current concepts in anatomy, biomechanics, clinical evaluation, imaging, prevention, and treatment strategies relating to finger flexor pulley system injuries. Our literature search was performed on PubMed with MeSH terms and keywords as subject headings to meet the objectives of this review. The "crimp grip" used in rock climbing is the mechanism for these injuries. The A2, A3, and A4 pulleys are at the highest risk of injury, especially when loaded eccentrically. Physical examination may reveal clinical "bowstringing," defined as the volar displacement of the flexor tendons from the phalanges; however, imaging is required for characterization of the underlying injury. Ultrasound is highly sensitive and specific for diagnosis and is recommended as the initial imaging technique of choice. Magnetic resonance imaging is recommended as an additional imaging study if ultrasound is inconclusive. Properly warming up increases the amount of physiologic bowstringing and is thought to prevent injury from occurring. Pulley injuries may be classified as grade I through IV. Conservative treatment, including immobilization, the H-tape method, and the use of a protective pulley splint, is recommended for grade I to III injuries. Surgical repair is reserved for grade IV injuries that are not amenable to conservative treatment.

Widening of Pulley by Interdigitating Cuts

While biomechanically important, pulleys present multiple challenges to a zone 2 flexor tendon repair and rehabilitation. Retrieval of tendon through an intact pulley is cumbersome. Following repair, venting is often needed to accommodate the bulk at repair site. On occasion, step lengthening, or even complete division of pulley and reconstruction with tendon graft are necessary. Finally, accessing the tendon under the pulley during tenolysis is difficult. The primary issue in all such cases is the lack of space under the pulley. We describe a technique of expanding the pulley by interdigitating cuts. This technique greatly increases the diameter of the pulley which could be demonstrated in cadaver model and noted in clinical setting. This technique is easy to perform and maintains the integrity of the pulley without complete division. It would allow easy tendon retrieval, provide space for the repair and gliding, and improve exposure during tenolysis.

Zone 2 Flexor Tendon Repair Location and Risk of Catching on the A2 Pulley

PURPOSE

To determine the region of the flexor digitorum profundus (FDP) and flexor digitorum superficialis (FDS) tendons in zone 2 that, when involved by a laceration repair, will reliably catch on the A2 pulley after surgery.

METHODS

Using fresh-frozen cadavers (5 hands, 20 digits), excursions of the FDP and FDS tendons were measured in relation to the A2 pulley. The C1, A3, and C2 pulleys were resected. The digit was maximally flexed by applying traction to the flexor tendon in the forearm. An 8-0 suture tag was placed in the flexor tendons immediately distal to the A2 pulley. The digit was then passively fully extended to measure tendon excursion. Measurements were repeated with 50% venting and 100% release of the A4 pulley. Reference points such as tendon insertions and flexion creases were obtained. This protocol was repeated sequentially for the index, middle, ring, and little fingers.

RESULTS

For all 20 fingers, the suture placed into the FDP just distal to the A2 pulley with the finger fully flexed traveled 1.6 ± 1.9 mm distal to the proximal edge of the A4 pulley with passive extension of the finger. The mean excursion for the FDP was 24.6 ± 3.2 mm, and 16.9 ± 3.1 mm for the FDS. The mean A2 pulley length was 16.2 ± 3.5 mm, and the mean distance between the distal edge of the A2 pulley and the proximal edge of the A4 pulley was 23.0 ± 3.3 mm. Venting the A4 pulley 50% and 100% increased FDP excursion a maximum of 0.9 and 1.9 mm, respectively.

CONCLUSIONS

An FDP repair proximal to the A4 pulley will slide under the A2 pulley with full active digital flexion after surgery. If the distal FDP stump lies underneath the A4 pulley with the digit fully extended, the FDP repair will not likely engage the A2 pulley with full flexion after surgery. The FDP excursion can be reliably predicted as a percentage of the A2 (distal) to the A4 (distal) pulley distance. Most importantly, the distance between the repair site and the A4 pulley approximately equals the length of the A2 pulley that requires release to avoid postoperative triggering.

CLINICAL RELEVANCE

Knowledge of this high-risk region of flexor tendon repair will guide surgeons regarding the potential need for partial release of the A2 pulley.

Biomechanics of the Acute Boutonniere Deformity

PURPOSE

To demonstrate which structures of the extensor mechanism create a boutonniere deformity, when damaged, in a cadaver model. An analysis of how damage to these anatomical structures affects the biomechanical performance of the extensor mechanism was also performed.

METHODS

We secured 18 fresh cadaveric hands onto an apparatus consisting of a computer-controlled motor and tensiometer attached in series to the extensor communis tendon of the ring and middle digits. The central slip, transverse, and oblique fibers of the interosseous hood and the triangular ligament were sequentially divided. After each structure was divided, the motors were activated to provide a constant tendon displacement force. The angular displacement at the proximal interphalangeal (PIP) and distal interphalangeal joints was recorded.

RESULTS

In all digits, detachment of the central slip from the middle phalanx produced a decrease in extension of the PIP joint. When the transverse and oblique fibers of the interosseous hood were also divided, extension at the PIP joint was further decreased. A boutonniere deformity occurred only when all 3 structures were damaged.

CONCLUSIONS

The boutonniere deformity requires subluxation of the lateral bands volar to the axis of rotation of the PIP joint. This study demonstrates that damage to the central slip alone does not cause the deformity. Combined injury of the central slip, triangular ligament, and transverse and oblique fibers of the interosseous hood causes a boutonniere deformity.

CLINICAL RELEVANCE

Division of the central slip leads to loss of extension at the PIP joint. A more substantial loss of extension after injury or development of a boutonniere deformity should alert clinicians that other structures of the extensor mechanism are also damaged.

Acute A4 Pulley Reconstruction with a First Extensor Compartment Onlay Graft

Background:

The integrity of the flexor tendon pulley apparatus is crucial for unimpaired function of the digits. Although secondary reconstruction is an established procedure in multi-pulley injuries, acute reconstruction of isolated, closed pulley ruptures is a rare occurrence. There are 3 factors influencing the functional outcome of a reconstruction: gapping distance between tendon and bone (E-space), bulkiness of the reconstruction, and stability. As direct repair is rarely done, grafts are used to reinforce the pulley. An advantage of the first extensor retinaculum graft is the synovial coating providing the possibility to be used both as a direct graft with synovial coating or as an onlay graft after removal of the synovia when the native synovial layer is present.

Methods:

A graft from the first dorsal extensor compartment is used as an onlay graft to reinforce the sutured A4 pulley. This technique allows reconstruction of the original dimensions of the pulley system while stability is ensured by anchoring the onlay graft to the bony insertions of the pulley.

Results:

Anatomical reconstruction can be achieved with this method. The measured E-space remained 0 mm throughout the recovery, while the graft incorporated as a slim reinforcement of the pulley, displaying no bulkiness.

Conclusions:

The ideal reconstruction should provide synovial coating and sufficient strength with minimal bulk. Early reconstruction using an onlay graft offers these options. The native synovial lining is preserved and the graft is used to reinforce the pulley.

Rehabilitation following zone II flexor tendon repairs

Ongoing clinical and basic research has improved understanding of flexor tendon mechanics and physiology for surgical repair and rehabilitation after a zone II flexor tendon repair. Yet, the ideal surgical repair technique that includes sufficient strength to allow safe immediate active motion of the finger, without excessive repair stiffness, bulk or rough surfaces resulting in excessive resistance to flexion, does not exist. After optimizing the repair, the surgeon and therapist team must select a rehabilitation plan that protects the repair but helps to maintain tendon gliding. There are 3 types of rehabilitation programs for flexor tendon repairs: delayed mobilization, early passive mobilization, or an early active mobilization. No guideline for rehabilitation should be followed exactly. Many factors influence therapy decisions, including repair technique, associated tendon healing, passive versus active range of motion, edema, and tendon adhesions. These factors can assist in guiding rehabilitation progression and promote functional range of motion, safely mobilize the repaired tendon(s) and prevent gapping, rupture, and adhesions.

Sonographical parameters of the finger pulley system in healthy adults

INTRODUCTION

To establish normative values of tendon to bone distances (TBDs) to evaluate the A2 and A4 annular pulley integrity, we hypothesized that these values correlate with gender, athletic exercise, occupation, individual's age and body height.

METHODS

Ultrasonography of 200 healthy individuals was performed prospectively. TBDs for the A2 and A4 pulley sections were measured for all fingers. Evaluation was performed in resting position and active forced flexion. Examination parameters included gender, age, body height, occupation, athletic exercise level, and hand dominance. Assessment of resting position and active forced flexion was done.

RESULTS

No clinically relevant differences of TBDs with respect to the aforementioned parameters were observed. But TBDs were significantly greater in active forced flexion than in resting position for all measured pulley sections. Intraobserver reliability was very satisfactory.

CONCLUSIONS

Establishing normative values will help to detect injured pulleys more precisely and examination should be performed both in resting position and active forced flexion.

Management of complications of flexor tendon injuries

Innovations in operative techniques, biomaterials, and rehabilitation protocols have improved outcomes after treatment of flexor tendon injuries. However, despite these advances, treatment of flexor tendon injuries remains challenging. The purpose of this review is to highlight the complications of flexor tendon injuries and review the management of these complications.

Flexor pulley system: anatomy, injury, and management

Flexor pulley injuries are most commonly seen in avid rock climbers; however, reports of pulley ruptures in nonclimbers are increasing. In addition to traumatic disruption, corticosteroid-induced pulley rupture has been reported as a complication of treating stenosing tenosynovitis. Over the last decade, there have been 2 new developments in the way hand surgeons think about the flexor pulley system. First, the thumb pulley system has been shown to have 4 component constituents, in contrast to the classic teaching of 3 pulleys. Second, in cases of zone II flexor tendon injury, the intentional partial A2 and/or A4 pulley excision or venting is emerging as a component for successful treatment. This is challenging the once-held dogma that preserving the integrity of the entire A2 and A4 pulleys is indispensable for normal digit function.