Biomechanical Evaluation of Flexor Tendon Function After Hamate Hook Excision

Transcription

1 Biomechanical Evaluation of Flexor Tendon Function After Hamate Hook Excision Fahir Demirkan, MD, Denizli, Turkey, James H. Calandruccio, MD, Denis DiAngelo, PhD, Memphis, TN Purpose: Hamate hook fractures are uncommon injuries for which treatment is controversial. Excision of the hamate hook is considered to be the preferred method of treatment but the effects of hamate excision are not clearly delineated. The purpose of this study was to determine what effect, if any, excision of the hamate has on flexor tendon function. Method: The biomechanical effects of hamate hook excision on flexor tendon function were studied in fresh cadaveric forearm specimens with wrists fixed in 3 positions (neutral, 30 extension, 30 extension with 30 ulnar deviation). Flexor tendon force, flexor tendon excursion, and flexor tendon shift were evaluated. Results: Flexor tendon force decreased after hamate hook excision (11% in neutral, 14% in 30 extension, and 15% in 30 extension with 30 ulnar deviation). The flexor profundus tendons had a 7- to 11-mm increase in proximal tendon excursion after hamate hook excision depending on the position of the wrist, and the flexor profundus tendons of the small finger shifted 4 to 5 mm in ulnar direction. Conclusions: The hamate hook provides some biomechanical advantage for flexor tendon function and cadaveric changes in tendon force after its excision suggest that power grip may be decreased after hamate hook excision. (J Hand Surg 2003;28A: Copyright 2003 by the American Society for Surgery of the Hand.) Keywords: Hamate hook, excision, excursion, flexor tendon force, ulnar shift. Hamate hook injuries are reported to comprise 2% to 4% of all carpal fractures. 1,2 Despite the rarity of these fractures in the general population they are quite common in athletes, especially baseball players, golfers, and tennis players. The diagnosis often is delayed because the signs and symptoms may be rather nonspecific and routine radiographs of the wrist are considered normal. 3,4 The standard treatment of symptomatic hamate hook fractures is surgical excision From the Department of Orthopaedic Surgery, Pamukkale University School of Medicine, Denizli, Turkey; and the Department of Orthopaedic Surgery, and the School of Biomedical Engineering, University of Tennessee-Campbell Clinic, Memphis, TN Received for publication April 11, 2001; accepted in revised form October 15, No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. Reprint requests: James H. Calandruccio, MD, Editorial Department, Campbell Foundation, 1211 Union Ave, Suite 510, Memphis, TN Copyright 2003 by the American Society for Surgery of the Hand /03/28A $35.00/0 doi: /jhsu because most fractures are diagnosed as chronic, established nonunions. 3 5 No anatomic model has been described previously to determine the mechanical changes that accompany hamate hook excision. Therefore the purpose of this study was to examine in a cadaveric model the biomechanical effects of hamate hook excision on flexor tendon force, flexor tendon excursion, and flexor tendon shift. Materials and Methods Eight normal, fresh-frozen cadaver forearms were fixed on a specially designed frame (Fig. 1). The wrist and the hand were secured to the frame platform by 3-mm K-wires passed through the radius and distal metacarpal shafts of the thumb and small finger metacarpals. Care was taken to not transfix or compress any flexor or extensor tendons. Flexor digitorum profundus (FDP) tendons were sectioned 8 cm proximal to the radiocarpal joint, near their musculotendinous junctions. The proximal ends of the long, ring, and THE JOURNAL OF HAND SURGERY 138

2 139 Demirkan, Calandruccio, and DiAngelo / Flexor Tendon Function Figure 1. Cadaveric forearm specimens were fixed on a custom frame assembly. Tendons were loaded with 5 lb or 10 lb of weight and small, ring, and long finger flexor tendon forces were measured with a load cell and connected to a recorder as shown. small finger FDP tendons were anchored together by No. 1 polypropylene sutures placed in modified Kessler stitch fashion. Via a standard open carpal tunnel incision temporary cardiac pacing wires made of multifilament surgical steel coated with blue polyethlene (2-0 Ethicon; Sommerville, NJ) were passed inside the epitenon sheath along the sides of the small finger FDP tendons. This incision also allowed later excision of the hamate hook. The hamate hook was excised through the carpal tunnel incision by extraperiosteal dissection to minimally disturb the transverse carpal ligament. The tendons were loaded with 5-lb (2.26 N) and 10-lb (5.33 N) forces and the flexor tendon forces in long, ring, and small fingers were measured with a button load cell (100-lb capacity, Cooper Instruments, Warrenton, VA) and displayed on a process panel meter (DFI Infinity CS, Cooper instruments). The forces were measured in 3 different wrist positions (neutral, 30 extension, and 30 extension with ulnar Figure 2. Flexor tendon excursions were measured as the distances between radiographic markers: tendon marker (m) and horizontal K-wire (arrowheads). Excursion changes were noted between (A) unloaded and (B) loaded states. Measurements also were made in various wrist positions both before and after hamate hook excisions.

3 140 The Journal of Hand Surgery / Vol. 28A No. 1 January 2003 Figure 3. Radiographs of flexor tendon shift. (A) The wire abuts the hamate hook when the tendon is loaded, (B) after excision of the hamate hook ulnar shift of the tendon was the measured distance between the wire and the radial margin of the hamate hook base (arrowheads). S, shift; W, wire. deviation). The forces were recorded both before and after hamate hook excision. The data obtained for flexor tendon force values were compared with each other by paired t-test (p.05). Tendon excursion changes were visualized radiographically on the AP plane and were recorded from images in loaded and unloaded positions (Figs. 2A,B) both before and after hamate hook removal. A radioopaque marker was placed on the tendons and sutured under the epitenon; excursion was measured as the change in distance of the marker from a K-wire drilled horizontally through the radius and ulna. Tendon excursions were recorded with a digimatic caliper (Mitutoyo Corporation, Arora, IL) and changes were evaluated with the t-test to evaluate for significance. The cardiac pacing wires within the tendons stayed in the same relative position throughout tendon movement. The distances of the wires from the radial margin of the hamate hook were recorded with a digital caliper. Ulnar shift of the flexor tendons was seen radiographically in the 3 different wrist positions when the tendons were loaded; both before and after hamate hook excision (Fig. 3). The measurements obtained before and after hamate hook removal were compared with each other with paired sample t-tests. Results Reductions in flexor tendon forces were found after hamate hook excision when the flexor tendons were loaded with 5-lb (2.26 N) and 10-lb (5.33 N) forces (Table 1). The changes of the flexor tendon forces varied with the wrist positions and with the individual fingers tested (Tables 2, 3; Figs. 4, 5). In general the percentage decrease in forces generated was enhanced with wrist extension and subsequent ulnar deviation. Respectively, under a 2.26 N load, 12.2% and 20% force reduction occurred in the small and ring fingers after hamate hook excision with the wrist in extension and ulnar deviation (Figs. 4, 5). Tendon excursions also varied with the wrist positions and hamate hook excision (Table 4). Hamate hook excision eliminated the bony radial pulley of the flexor tendons and allowed the tendons to shift ulnar- Table 1. Reductions in Flexor Tendon Force After Hamate Hook Excision 5-lb (2.26 N) Force 10 lb (5.33 N) Force Small finger 8% to 12% 7% to 9% Ring finger 11% to 20% 11% to 20% Long finger 12% to 16% 12% to 16%

4 141 Demirkan, Calandruccio, and DiAngelo / Flexor Tendon Function Table 2. Tip-to-Palm Flexor Tendon Forces (N) / SD When 5-lb (2.26 N) Forces Are Applied to FDP Tendons With and Without Hamate Hook Excisions in Various Wrist Positions Wrist Position Neutral 30 Extension 30 Extension Ulnar Deviation Small finger FDP Hamate hook intact 6.2 / / / 0.9 Hamate hook excised 5.6 / / / 1.0 p values % drop Ring finger FDP Hamate hook intact 4.0 / / / 1.4 Hamate hook excised 3.6 / / / 1.1 p values % drop Long finger FDP Hamate hook intact 4.5 / / / 0.7 Hamate hook excised 3.9 / / / 0.7 p values % drop ward. This slackening of the flexor tendons was most notable when wrists were ulnarly deviated and resulted in a 26% increase in common flexor tendon proximal excursion of the long, ring, and small fingers. The FDP and flexor digitorum superficialis tendons of the fifth finger were the closest tendons to the hamate hook. Radiographic markers along the small finger FDP tendon were 2 to 3 mm away from the radial margin of the hamate hook at rest. When the tendon was loaded the FDP abutted against the hamate (Fig. 3A). When the tendons were loaded with the wrists fixed in neutral position the tendon was 5 to 6 mm away from hamate radial margin, and when the hamate hook was removed the FDP marker shifted 8 to 9 mm ulnarward from the hamate radial margin (Fig. 3B). With wrists in extension and ulnar deviation the tendon displacements ranged between 11 to 12 mm (Table 5). Discussion Hamate hook fractures in athletes may be divided into direct and indirect injuries. 6 Direct mechanisms of injury include acute impingement on the hamate hook by a handle such as that of a bat, racket, or golf club, and the indirect mechanism refers to shearing fractures of the hamate hook base by flexor tendons contracting Table 3. Tip-to-Palm Flexor Tendon Forces (N) / SD When 10 lb (4.53 N) Forces Are Applied to FDP Tendons With and Without Hamate Hook Excisions in Various Wrist Positions Wrist Position Neutral 30 Extension 30 Extension Ulnar Deviation Small finger FDP Hamate hook intact 11.9 / / / 1.8 Hamate hook excised 11.2 / / / 1.8 p values % drop Ring finger FDP Hamate hook intact 7.8 / / / 2.7 Hamate hook excised 6.9 / / / 2.2 p values % drop Long finger FDP Hamate hook intact 8.7 / / / 1.4 Hamate hook excised 7.6 / / / 1.4 p values % drop

5 142 The Journal of Hand Surgery / Vol. 28A No. 1 January 2003 Figure 4. Tip-to-palm flexor tendon forces (N) when 5-lb (2.26 N) forces were applied to FDP tendons with the wrist in 3 positions with the hamate hook intact and excised. ext, extension; ext-ud, extension and ulnar deviation. s, hamate intact;, hamate excised. forcefully as they move ulnarly in a power grip. 6 Patients usually present late and complain of increasing pain at the dorsal and ulnar aspects of the hand with use of the hand. Symptoms are more frequent during activities requiring grip. Hamate hook nonunions frequently impair function and may result in delayed flexor tendon rupture 7 and ulnar nerve dysfunction. 2 Relief of pain and return to preinjury function are reported in patients with simple excision of the hamate hook. The complication from excision itself is less than 2%. 5 Although patients symptoms are reported to resolve after surgery, grip strength measurements are not documented. 3,4,8 11 Moreover the biomechanical effects of hamate excision have not been studied. Bone grafting of hamate hook nonunions is recommended by some investigators who claim advantage in preserving the hamate hook pulley effect on flexor tendons. Watson and Rogers 6 reported that grip returned to normal levels after surgery; however, their Figure 5. Tip-to-palm flexor tendon forces (N) when 10-lb (5.33 N) forces were applied to FDP tendons with the wrist in 3 positions with the hamate hook intact and excised. ext, extension; ext-ud, extension and ulnar deviation. s, hamate intact;, hamate excised. results indicated a 7% to 17% decrease of grip strength when compared with the uninjured hand. Open reduction and internal fixation with screws and K-wires have likewise been advocated in several small series of hamate hook fractures. 7,12,13 Fixation of such a small fragment is difficult 2 and there is a risk for avascular necrosis and nonunion. 14,15 The flexor tendons lie inside the carpal tunnel and both the transverse carpal ligament and the hamate hook act as a pulley preventing ulnar shift and bowstringing of the flexor tendons. Releasing the carpal tunnel results in a 6% excursion change of the FDP tendons with the wrist fixed in neutral position. 16 There was a 15% proximal excursion of the tendons after hamate hook excision in the neutral wrist owing to the ulnar shift and subsequent laxity of the flexor tendons. Bowstringing of the flexor tendons still occurred despite leaving the portion of the transverse carpal tunnel ligament attaching to the pisiform intact. Table 4. Excursions of the Flexor Tendons (mm) Measured RadiographicallyWhen Loaded With 5 lb (2.26 N) Forces With and Without Hamate Hook Excisions in Various Wrist Positions Wrist Position 30 Extension Excursion (mm) (n 8) Neutral 30 Extension Ulnar Deviation Hamate hook intact (mm) Hamate hook excised SD (mm) Change (mm) Change (%) p values

6 143 Demirkan, Calandruccio, and DiAngelo / Flexor Tendon Function Table 5. Shifts of the Flexor Tendons AwayFrom the Radial Border of the Hamate Hook With and Without Hamate Hook Excision in Various Wrist Positions Wrist Position Shift (mm) (n 8) Neutral 30 Extension 30 Extension Ulnar Deviation Unloaded (mm) Hamate hook intact (mm) Hamate hook excised SD (mm) Change (mm) p values Our experimental study showed that after hamate hook excision there was a reduction of flexor tendon force owing to the loss of the normal hamate pulley. If we combine the forces applied by each finger we can assume that there is an average 11% reduction of grip strength with the wrist in neutral wrist position, 14% in extension, and 15% in extension and ulnar deviation. Although not included in our study, alteration of the normal origins of the hypothenar intrinsic may further decrease grip strength. The structural changes that occur in vivo after hamate hook excision are not known. Certainly remodeling of the ulnar and distalulnar boundaries of the carpal tunnel will act to restrain ulnar shift and bowstringing of the flexor tendons. Moreover length-tension relationship adaptation of the effectively lengthened flexor tendon musculotendinous units may compensate for the acute biomechanical losses we found in our study. Results from this cadaver study, however, may help explain persistent weakness or loss of complete composite digital flexion after hamate hook excision. The authors would like to thank Janice Hepler, Executive director of Medical Education & Research Institute, Memphis, TN, and Richard Smith, Assistant Professor, Orthopedic Research Laboratory, for their contributions. References 1. Walsh JJ, Bishop AT. Diagnosis and management of hamate hook fractures. Hand Clin 2000;16: Boulas HJ, Milek MA. Hook of the hamate fractures. Diagnosis, treatment, and complications. Orthop Rev 1990;19: Carter PR, Eaton RG, Littler JW. Ununited fracture of the hook of hamate. J Bone Joint Surg 1977;59A: Stark HH, Jobe FW, Boyes JH, Ashworth CR. Fracture of the hook of hamate in athletes. J Bone Joint Surg 1977;59A: Smith P, Wright TW, Wallace PF, Dell PC. Excision of the hook of the third hamate: a retrospective survey and review of the literature. J Hand Surg 1988;13A: Watson HK, Rogers WD. Nonunion of the hook of hamate: an argument for bone grafting the nonunion. J Hand Surg 1989; 14A: Milek MA, Boulas HJ. Flexor tendon ruptures secondary to hamate hook fractures. J Hand Surg 1990;15A: Bishop AT, Beckenbaugh RD. Fracture of the hamate hook. J Hand Surg 1988;13A: Stark HH, Chao EK, Zemel NP, Rickard TA, Ashworth CR. Fracture of the hook of hamate. J Bone Joint Surg 1989;71A: Futami T, Aoki H, Tsukamoto Y. Fracture of the hook of hamate in athletes. Eight cases followed for 6 years. Acta Orthop Scand 1993;64: Foucher G, Schuind F, Merle M, Brunelli F. Fractures of the hook of hamate. J Hand Surg 1985;10B: Blair WF, Kilpatrick WC, Omer GE. Open fracture of the hook of the hamate. A case report. Clin Orthop 1982;163: Khan MI. Fracture of the hook of hamate. Letter to the editor. J Hand Surg 1986;11A: Failla JM. Hook of hamate vascularity: vulnerability to osteonecrosis and nonunion. J Hand Surg 1993;18A: Failla JM. Osteonecrosis associated with nonunion of the hook of the hamate. Orthopedics 1993;16: Kang HJ, Lee SG, Phillips CS, Mass DP. Biomechanical changes of cadaveric finger flexion: the effect of wrist position and of the transverse carpal ligament and palmar and forearm fasciae. J Hand Surg 1996;21A: