Introduction
The majority of flexor tendon injuries in Zone 1 are caused by closed avulsion or laceration [1]. Moiemen and Elliot [2] classified Zone 1 into subzones based on the differences in surgical approaches required depending on the injury location (Fig. 1).
Historically, the pullout button technique, as described by Bunnell [3], has been employed for treating complete divisions of the flexor digitorum profundus (FDP) tendon in the distal portions of Zone 1a and 1b. However, this technique has been associated with various complications, including nail deformity, hypersensitivity, skin necrosis, and infection, leading to the introduction of alternative methods such as suture anchors (Fig. 2 [3–8]) [1,2,9–12].
These alternative methods were developed primarily because of the difficulty of performing core sutures near the FDP insertion site, where the tendon is often divided [1,2,9]. Malerich et al. [13] demonstrated through cadaver studies that when the distal stump of a divided tendon is less than 1 cm from the insertion site, advancing the proximal stump and securing it to the bone does not result in the quadriga effect or flexion contracture.
Consequently, advancing the FDP tendon and attaching it to the bone has become a preferred treatment when the division occurs within 1 cm of the insertion site [9]. Murphy and Mass [1] suggested that a minimum length of approximately 0.75 cm of the distal stump is required for effective locking sutures.
This paper reports the outcomes of tendon-to-tendon repairs using the loop-locking suture technique for cases in Zone 1a and Zone 1b, where the FDP tendon was completely divided and the distal stump was less than 0.8 cm in length.
Methods
Ethics statement: The study was approved by the Institutional Review Board of Gwangmyeong Sungae Hospital (No. KIRB-2024-N-010) and performed in accordance with the principles of the Declaration of Helsinki. Written informed consent was obtained from all the patients.
1. Subjects
This study included patients who underwent complete division of the FDP tendon between June 2011 and January 2024, in which the length from the insertion site to the tendon stump was less than 0.8 cm. These cases correspond to the distal parts of Zone 1a and Zone 1b according to the Moiemen’s classification [2]. The insertion site of the FDP tendon is defined as the most proximal point where the tendon fibers attach to the distal phalanx [13]. Patients with fractures (four cases), volar plate injuries (one case), avulsion injuries (two cases), and one patient with colon cancer who was not in good general health for proper evaluation were excluded from the study. Of the 21 patients treated during this period, 13 were available for follow-up, with an average postoperative follow-up period of 12 months (Table 1). The patients’ ages ranged from 26 to 68 years, with a mean age of 47.9 years, and included 10 males. The affected digits were 10 index fingers, two middle fingers, and two little fingers. In seven cases, the injury involved the dominant hand. Ten cases had associated nerve and vascular ruptures, while four cases involved only tendon division. The mean length of the tendon stump from the insertion site to the proximal end was 0.61 cm (range, 0.4–0.8 cm).
2. Surgical methods
All 13 cases were operated under brachial plexus block anesthesia. To expose the proximal and distal ends of the divided tendon, a Bruner zigzag incision, midlateral incision, or minimal incision at the distal pulp was made. In nine cases, the proximal tendon was located distal to the A2 pulley and was exposed through the wound by passing it through the A4 pulley using forceps. In four cases, the proximal tendon was located at the A1 pulley, requiring an incision at the distal palmar crease to expose the divided tendon, which was then retrieved to the wound site using a Carroll tendon retriever. The exposed proximal tendon was fixed with a 23-gauge needle. A polydioxanone (PDS) or poliglecaprone 25 4-0 suture was passed through the tendon fibers adjacent to the bone surface, aiming to capture as many fibers as possible near the distal phalanx. A loop was then formed at the entry point of the needle outside the tendon. The suture was subsequently passed transversely through the tendon, creating a single locking loop at the surface, before being brought back through the initial entry point. This loop provided sufficient tensile strength by securely holding the fan-shaped tendon insertion site, even with a relatively short tendon length. The same loop and locking techniques were applied to the proximal tendon. A two-strand core suture was completed by tying knots inside the tendon using PDS 4-0. Continuous epitendinous suturing was performed without cutting the suture. Specifically, one locking suture was applied to each of the proximal and distal tendon ends, and a loop was used for both ends (Fig. 3).
In cases where the digital artery and nerve were ruptured, the artery was repaired under a microscope using nylon 10-0, and the nerve was sutured using interrupted epineural sutures with nylon 10-0. For immobilization and protection, the wrist was placed in a neutral position, the metacarpophalangeal joint was flexed to 80° to 90°, and the interphalangeal joints were kept in extension with a dorsal splint. The day after surgery, gentle active movement of the fingers was encouraged while the splint was in place.
3. Postoperative evaluation
Passive and active joint angles were measured dorsally for each joint using the American Medical Association method. Flexion contracture was recorded as extension lag. The analysis followed Strickland’s criteria [14], which calculates the percentage by subtracting the extension lag of both the proximal and distal interphalangeal joints (DIPJs) from their combined range of motion (ROM), then dividing by the normal average ROM of 175°. Strickland’s classification defines the following categories: excellent (85%–100%, 150°–175°), good (70%–84%, 125°–149°), fair (50%–69%, 90°–124°), and poor (0%–49%, 0°–90°) [14].
Additionally, Moiemen’s classification was used for further analysis of the DIPJ in Zone 1 tendon injuries, excluding the proximal interphalangeal joint angles. In this method, the ROM of the DIPJ minus the extension lag is divided by 74° and expressed as a percentage. The categories are defined as follows: excellent (85%–100%, 62°–74°), good (70%–84%, 51°–61°), fair (50%–69%, 37°–50°), and poor (0%–49%, 0°–36°) [2]. Grip strength, key pinch, and pulp pinch were measured using a Jamar dynamometer, and the results were expressed as a percentage by comparing the injured side with the contralateral uninjured side [11].
Results
No tendon re-rupture was observed. The average active ROM of the DIPJ was 61.5° (range, 45°–75°). Grip strength, compared to the uninjured side, was 95.3%, key pinch strength was 98.5%, and pulp pinch strength was 86.8%. According to Strickland’s criteria [14], 10 cases were rated as excellent and three cases as good. Using Moiemen’s classification [2], five cases were rated as excellent, five as good, and three as fair. Flexion contracture was observed in two cases, with an average of 15°. All two patients sustained injuries from mechanisms that caused the crushing of both tendon ends, such as electric saws and grinders. This resulted in the loss of the tendon edge, which was subsequently repaired after debridement. However, the quadriga effect was not observed. In two cases, tenolysis was performed due to tendon adhesion at Camper’s chiasm, improving the active ROM of the DIPJ from 35° and 40° preoperatively to 75° postoperatively in both cases (Tables 2, 3). Only two patients underwent physical therapy starting from the 4th postoperative week, while the other patients declined therapy, reporting no discomfort in using their fingers. Therefore, an analysis of the effects of physical therapy could not be conducted. The splint was removed 5 weeks after surgery.
1. Case 1
A 26-year-old male patient presented with a laceration of the right middle finger caused by a knife. The wound was a clean-cut laceration, and the patient was unable to actively flex the DIPJ. Surgery was performed 4 hours after the injury. The FDP tendon was completely divided 0.5 cm proximal to its insertion, with the proximal end of the divided tendon located distal to the A2 pulley. Intraoperative exploration under a microscope revealed no damage to the digital nerves or arteries. A Bruner zigzag incision was made along the radial side of the wound to expose the distal stump of the divided FDP tendon. The proximal stump, which was caught at the A2 pulley, was then pulled distally and fixed using a straight needle. It was subsequently passed through the A4 pulley and brought to the wound site, where a two-strand locking loop suture and epitendinous sutures were placed using a PDS 4-0 (Fig. 3).
Active finger movement was initiated on the first postoperative day while the splint was still in place, and the splint was removed 5 weeks postoperatively. Twelve months postoperatively, the active ROM was 100° at the proximal interphalangeal joint and 65° at the DIPJ. According to Strickland’s criteria and Moiemen’s classification, the results were rated excellent. Grip strength was 105% compared to the contralateral side, pulp pinch strength was 78%, and key pinch strength was 106% (Figs. 4, 5).
2. Case 2
A 32-year-old female patient presented with a laceration to the left index finger caused by scissors. The wound was a clean-cut laceration, and the patient was unable to actively flex the DIPJ. Surgery was performed 2 hours after the injury. The FDP tendon was completely divided 0.6 cm proximal to its insertion, with the proximal stump located distal to the proximal interphalangeal joint. There was also a rupture of the radial digital nerve and artery. A central incision was made from the center of the wound to the distal pulp to expose the distal stump of the divided FDP tendon. The proximal stump, which was caught at the A4 pulley, was then passed through the A4 pulley and brought to the wound site, where it was sutured using the same technique as in the previous case. The digital nerve and artery were repaired under a microscope using nylon 10-0 sutures.
Active finger movement was initiated on the first postoperative day while the splint was still in place, and physical therapy was started 4 weeks postoperatively. However, the patient was sensitive to pain and had poor compliance with the finger exercises. At 4 months postoperatively, the active ROM of the proximal interphalangeal joint was 80°, and the active ROM of the DIPJ was 60°.
At 20 months post-injury, active ROM improved to 100° at the proximal interphalangeal joint and 75° at the DIPJ. According to Strickland’s criteria and Moiemen’s classification, the results were rated excellent. Grip strength was 100% compared with the contralateral side, pulp pinch strength was 80%, and key pinch strength was 105%, all of which showed good outcomes. Additionally, the two-point discrimination test showed normal values of 5 mm for static discrimination and 3 mm for dynamic discrimination (Fig. 6).
Discussion
The DIPJ plays a crucial role in tasks such as power gripping, where a large object like a coffee jar lid needs to be gripped tightly, and in span pinch, where fingers are extended to grasp objects [2]. It is particularly important in the index and middle fingers for fine pinch tasks, which require delicate manipulation of objects [2]. Evans reported that a ROM of approximately 40° is necessary for the proper function of the DIPJ [15,16]. However, it has been observed that patients with Zone 1 deep flexor tendon injuries are often dissatisfied even if they achieve more than 40° of flexion in the DIPJ without any flexion contracture in the proximal interphalangeal joint [1,2,17]. Due to the limited space in Zone 1, tendon injuries in this area must carefully consider the potential for tendon bunching, which increases tendon diameter and subsequently hinders tendon gliding—a key concern in the treatment of these injuries [2].
Since Bunnell [3] introduced the use of stainless-steel pullout wires in 1948 for Zone 1 tendon injuries, various modifications have been utilized, including those by Planas [18], as well as tendon grafting techniques by Pulvertaft [19], and external fixation methods using a fishmouth incision at the fingertip by Sood and Elliot [6]. Additionally, methods such as suture anchor by Hallock [4] have been applied not only for tendon avulsion injuries but also for treating tendon ruptures in Zone 1 (Fig. 2) [1,2,9-12,20,21].
Several authors have recognized the drawbacks of Bunnell’s technique and have sought to overcome them. Teo et al. [10] highlighted that the Bunnell’s pullout button technique and its modifications can lead to complications such as button loosening, infection from external fixation, and nail deformities. Pulvertaft’s transosseous reinsertion method [19], is technically difficult and risks fracturing the bone. Sood and Elliot’s [6] can cause fingertip hypersensitivity and the quadriga effect, while the suture anchor method risks misplacement and bone fractures. Moreover, the suture anchor technique is also noted to be expensive [22]. Kang et al. [21] studied 23 patients with Zone 1a injuries treated with the pullout button technique, reporting complications in 15 cases. These included nail growth issues (eight cases), infections (five cases), hypersensitivity (three cases), pain (one case), and tendon re-rupture (one case). Hargreaves et al. [23] also noted that infection rates rise with longer Kirschner wire retention, which applies to the pullout button technique.
In cases of complete FDP tendon division at the insertion, most surgical methods involve attaching the tendon to the bone [1,2,9-12,17,20,21]. Boyer et al. [9] and Strauch [24] found that reattaching the tendon increases osteoclast activity, significantly reducing bone density. In an animal study, bone density decreased by up to 40% within 6 weeks [11]. Silva et al. [20] observed minimal improvement in strength and rigidity with the pullout button technique, concluding that tendon fixation to the bone delays healing [9,20]. In another cadaver study, Silva et al. [12] observed that during active flexion of the digit, a force of 20 N applied to the FDP tendon caused a gap of over 2 mm between the tendon and bone, suggesting that this method may not provide sufficient resistance to forces encountered during early rehabilitation. Leversedge et al. [25] identified that the region within 1 cm of the FDP insertion site is a hypovascular zone, indicating that tendon repair in this area may not heal well when the tendon is fixed to the bone.
McCallister et al. [11] reported the results of 26 patients with complete FDP tendon division or avulsion in Zone 1, comparing 13 patients treated with the pullout button technique and 13 treated with the suture anchor technique. They measured ROM and grip strength and compared the injured and uninjured sides. In the pullout button group, the average ROM of the DIPJ was 57.31°, and the grip strength was 97.76%. In the suture anchor group, the average ROM was 56.54°, and the grip strength was 101.96%. The results from our institution showed an average ROM of 61.5° in the DIPJ and grip strength of 95.3% compared with the uninjured side with an average ROM of 61.1° and grip strength of 95.8%.
In cases of complete FDP tendon division in Zone 1a, where the remaining length of the distal stump is short, it has been assumed that core sutures or locking sutures would be difficult to apply and that simple tendon suturing alone would not provide sufficient tensile strength. Therefore, methods such as the pullout button or suture anchor technique, which involves fixing the tendon to the bone, have been commonly used. However, even though all distal stumps were as short as 0.8 cm or less, we still achieved satisfactory results using the loop-locking suture technique for tendon-to-tendon repair.
The ultimate tensile strength at the insertion site of the FDP tendon is 118.6±12.8 N [12], which is lower than the average ultimate strength of 213.0±12.0 N found in the midportion of the FDP tendon (Zone II) [26]. Although the four-strand core suture provides greater tensile strength, the relatively lower tensile force required at the FDP insertion site suggests that a two-strand core suture is sufficient. Additionally, at the insertion site, where the tendon spreads out and becomes thinner as it attaches to the bone, achieving sufficient tensile strength through tendon suturing is challenging owing to the short distance and fan-shaped configuration of the tendon. Techniques like the pullout button or suture anchor, known for providing high tensile strength, are commonly recommended. However, they come with various complications as previously mentioned.
A single locking suture is considered and applied in clinical practice to achieve adequate tensile strength within the insertion site while minimizing gap formation. This approach enhanced the tensile strength of the two-strand core suture, allowing secure fixation of the thin tendon, thus facilitating early active motion by reducing gap formation. The key to this technique is the application of the loop-locking suture to the distal portion. This study included patients with a distal stump ranging from 0.4 to 0.8 cm. Since the distal phalanx provides support for tissue fixation during surgery, it eliminates the need to grasp the tendon directly. We believe that our technique could be applied even with a tendon length shorter than 0.4 cm for suturing, especially because the suture technique itself is simple and adaptable.
Many authors have described the advantages of locking sutures. Hotokezaka and Manske [27] found that, compared to grasping sutures, single locking sutures in two-strand core sutures provided the highest tensile strength and minimized gap formation. Lee et al. [28] noted that uninjured flexor tendons have a tensile strength of 900 to 1,500 g under resistance, while Verden and Michon [29] found that Kessler sutures, with a critical tension of 1,250 g, often exhibit gap formation under a continuous force of 1,000 g. Lee [28] found that four-strand double loop-locking sutures had a tensile strength of 4,400 g compared to 2,250 g for Kessler sutures, with significantly less gap formation, making them suitable for early active motion. Given the thinness of the FDP tendon at the insertion site and the short length of the divided distal stump, the authors opted for a simpler technique by applying a single loop and locking suture to both the proximal and distal ends instead of using the more complex double loop-locking suture.
A recently introduced technique similar to the two-strand loop-locking suture method used at our institution is the two-strand side-locking loop technique (Fig. 7), described by Kuwata et al. [26] noted that this method is a good choice for suturing thin tendons. They conducted an experiment on pig FDP tendons (corresponding Zone 2 in humans) and found that under 50 N load, the average gap was 1.2±0.8 mm, with a tensile strength of 207.1±15.2 N, demonstrating that the small gap and high tensile strength enabled early active motion. Noguchi et al. [30] reported that the average ultimate tensile strength is 213.0±12.0 N. The loop-locking suture method we use at our institution is simpler than the two-strand side-locking loop technique, making it more practical for repairing tendon ruptures at the FDP insertion site, where suturing is more challenging.
At our institution, a two-strand core suture was performed using PDS 4-0 or Monocryl 4-0, and continuous epitendinous suturing was performed without cutting the PDS 4-0 suture. There were no issues with the suture damaging the tendon or causing complications, such as granuloma formation.
The limitations of this study include the small number of cases with complete FDP tendon division at the insertion site without associated fractures or volar plate injuries, as well as the limited number of patients who were available for follow-up. A minimum length of the tendon stump is required to perform the suture, which can be another limiting factor. Additionally, the effectiveness of postoperative physical therapy could not be fully assessed. Further biomechanical studies on the tensile strength of the tendon suture site are needed to evaluate the effectiveness of the loop-locking suture technique.
Conclusion
Tendon-to-tendon repairs performed at our institution for Zone 1a and Zone 1b injuries, where the FDP tendon was completely divided near the insertion site with a distal stump length of less than 0.8 cm, demonstrated satisfactory functional recovery without complications, such as re-rupture or infection. These outcomes were comparable to those achieved with commonly used methods, such as the pullout button external fixation and suture anchor bone fixation.
The loop-locking suture technique used for tendon-to-tendon repair is relatively simple to perform, with low morbidity and a reduced incidence of complications. It does not incur the costs associated with suture anchors and allows for the tendon to be positioned in its normal anatomical location. Therefore, in cases where the distal stump of the FDP tendon is less than 0.8 cm in length at the insertion site, the loop-locking suture technique offers a viable alternative to traditional advancement and bone fixation methods.
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