Anatomic relationships of an endoscopic carpal tunnel device to surrounding structures

Transcription

1 This is" a reprint of an article in The Journal of Hand Surgery (American Volume) as it was published in its original foi m. Articles in The Journal are not prepared for any company or distributor, and publication of any article therein does not constitute an endorsement by the Publisher, Editor, or ASSH of any product described therein. Anatomic relationships of an endoscopic carpal tunnel device to surrounding structures Anatomic relationships of an endoscopicarpal tunnel device to surrounding soft tissue structures along the ring finger and the long-ring interspace axis were investigated in 28 adult cadaver hands. The average distance from the center of the device to the media nerve in the carpal tunnel averaged 3.3 mm in the ring finger axis and 2.5 mm in the long-ring interspace axis. The average distance from the distal edge of the transverse carpal ligament to the superficial palmar arch was 4.8 mm in the ring finger axis and 5.5 mm in the long-ring interspace axis. These and other more subtle anatomic observations indicate the greater safety of using the ring finger axis for endoscopi carpal tunnel release. (J HAND SURG 1993;18A: ) Mitchell B. Rotman, MD, and Paul R. Manske, MD, St. Louis, Mo. The anatomy of the carpal tunnel has been well described. 13 With the recent development, increased popularity, and reports of complications of endoscopic carpal tunnel surgery, 4I the relationships to adjacent soft tissue structures in the region of the release is of extreme importance. The purpose of the present study was to define these anatomic relationships. ~ From the Division of Orthopedic Surgery, Washington University School of Medicine, St. Louis, Mo. Received for publication Aug. 23, 1991; accepted in revised form July 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: Mitchell B. Rotman, MD, Division of Orthopedic Surgery, Washington University School of Medicine, West Pavilion, One Barrtes Hospital Ptaza, St. Louis, MO /1/42893 Materials and methods Twenty-eigh t cadaver hands were studied. In 24 paired adult cadaver hands, a transverse incision was made in the palmar wrist crease between the palmaris longus and the flexor carpi ulnaris tendons, identifying the anterior forearm fascia and the proximal edge of the transverse carpal ligament (TCL). Through an opening in the anterior forearm fascia, the carpal tunnel was entered; the ulnar bursa, adjacent to. the radial border of the hook of the hamate, was dissected off the overlying TCL with a Freer elevator. A cannulated pathfinder probe was passed through the carpal canal, along the direction of either the ring finger (RF) axis or the long-ring interspace (LRI) axis. A guide wire was then placed through the probe, and the probe was removed. An Inside Job blade assembly housing (Orthopedic Products Division, 3M Health Care, St. Paul, Minn.), with the tip removed, was placed ov~er the guide wire (Fig. 1).. The guide wire was removed, leaving the 6 mm device in place. 442 THE JOLYRNAL OF HAND SURGERY

2 Vol. 18A, No. 3 May t993 Endoscopic carpal tunnel device 443 Fig. 1. Blade assembly housing (below) and modification of blade assembly housing for cannulation with guide wire (above). Fig. 2. A, Device placed within carpal canal against radial border of hook of hamate and directed along RF axis. Markings on the palm show the RF axis and the hook of hamate. B, Device placed along LRI axis. The specimens were divided into two groups. In group A (14 specimens) the device was directed along the RF axis (Fig. 2, A); in group B (10 specimens), the device was directed along the LRI axis (Fig. 2, B). The prepared hands were frozen at - 70 C to maintain the specific position of the device during sectioning. Four additional hands that had not been operated on were prepared for composite sectioning. The anatomic relationships investigated were (1) the distance from the center of the device (corresponding to the position of the endoscopic blade) to the median nerve (transverse sections), (2) the location of the rounding flexor tendons relative to the device (transverse sections), (3) the distance from the distal margin of the TCL to the superficial palmar arch (SPA) and the fat that invests the SPA and common digital nerves (sagittal sections), and (4) the interface between TCL and the more superficial palmar fascia and sub-

3 444 Rotman and Manske The Journal of HAND SURGERY Fig. 3. Specimen transversely sectioned at 1 cm intervals, starting at palmar wrist crease, showing device in proximal carpal tunnel. The medianerve (circle) is seen just radial to the device. P, Pisiform; S, scaphoid. Fig. 4. Specimen sagittally sectioned in line with RF axis and bisecting device. cutaneous structures (composite and transverse sections). Transverse sections. Nine specimens (five in group A, four in group B) were transversely sectioned in the frozen state with a vertical band saw at 1 cm intervals, starting at the level of the palmar wrist crease and continuing distally to divide the tunnel into proximal, middle, and distal regions (Fig. 3). Measurements to the nearest 0.5 mm were made in each of the three regions under x 6.3 to x 10 magnification (Photomakroscop M-400, Wild Microscopes, Division of Leica, Inc., Rockleigh, N.J.). The distance from the center of the device (i.e., the position of the endoscopic blade) the median nerve was recorded. Since the edge of the

4 Vol. 18A, No. 3 May 1993 Endoscopic carpal tunnel device 445 Table I. Transverse sections Distance (mm) from center of device to median nerve the carpal tunnel Group A (RF axis) Proximal ( ) Middle 3.0 ± 0.0 Distal ( ) Group B (LRI axis) Proximal ( ) Middle 2.5 ± 1.3 ( ) Distal 1.8 ± 1.7 ( ) Values are mean ~ standardeviation, with range in parentheses. Table II. Sagittal sections Distance (mm) from distal edge of TCL to SPA Group A 4.8 ± 0.8 ( ) Group B 5.5 ± 0.7 ( ) Distance (mm) from proximal extent of fat pad distal edge of TCL Group A 2.8 ± 0.6 ( ) Group B 0.8 ± 1.1 ( ) Values are mean +- standard deviation with range, in parentheses... device was 3 mm from the center, a measurement of <3 mm indicated that the nerve was between the device and the overlying TCL. A negative measurement indicated that the nerve extended ulnarly beyond the center of the device (i.e., between the position of the blade and the TCL). Sagittal sections. Fifteen specimens (nine in group A, six in group B) were sagittally sectioned along the RF or LRI axis, bisecting the device (Fig. 4). The distances to the nearest 0.5 mm were measured from the distal margin of the TCL to the SPA, as well as to the proximal extent of the investing fat pad. Composite sections. A composite section of four specimens, including skin, the TCL, and the interposed soft tissue structures, was obtained along the long finger (LF), LRI, and RF axes (Fig. 5). Longitudinal histologic sections n 12 were obtained along each axis. The longitudinal palmar fascia fibers were seen in the sagittal plane, whereas the TCL fibers were seen in the coronal plane. Statistical analysis. Statistical analysis was computed by means of PC analysis of variance statistical analysis software (Human Systems Dynamics, Northridge, Calif.). Statistical significance was defined as p value <0.05. All p values were computed by means of analysis of variance for a simple randomized design with unweighted means. Fig. 5. Hand specimen showing where composite section has been obtained. Results Transverse sections (Table I). In group A (RF axis), the nerve was positioned along the side of the device (Fig. 6, A). The average distance from the center the device to the median nerve in the proximal carpal tunnel was 3.8 ram; in the midcarpal tunnel region the distance was 3.0 mm; and in the distal carpal tunnel the distance measured 3.1 mm. In no instance was the nerve found between the device and the TCL. In group B (LRI axis), when the nerve was positioned along the side of the device, it appeared flattened (Fig. 6, B). The average distance from the center of the device to the nerve in the proximal carpal tunnel measured 3.1 mm. In the midcarpal tunnel, the distance measured 2.5 ram, and in the distal carpal tunnel the distance measured 1.8 mm; this placed the nerve between the device and the TCL. The nerve was found between the device and the TCL in 20% of the specimens; in one instance the nerve would have been under the blade.

5 446 Rotman and Manske The Journal of HAND SURGERY Distance from Center of Device to Median Nerve C Proximal Middle Distal Region Fig. 6. A, Transverse section of distal carpal tunnel along RF axis showing device along undersurface of TCL between hamate (H) and blackened median nerve (M). The nerve is directly alongside, but not pressed against, the device. B, Media nerve (M) pressed against device along LRI axis. (Original magnification x 6.3.) C, Distances from center of device to media nerve in RF and LRI axes are compared. The differences are statistically significant in the proximal (p 0.03) and distal (p 0.04) carpal tunnel regions. D, In this specimen, the device was inadvertently rotated toward media nerve. Note how the device conforms to the curve of the lateral border of the hamate (H). The importance of placement of the device snugly against the palmar surface of the TCL is apparent. The nerve (M) is between the center of the device and the TCL. (Original magnification x 6.3.) The differences at the proximal and distal regions are significantly different between the two axes as noted in Fig. 6, C. In one specimen the device was inadvertently rotated toward the median nerve (Fig. 6, D). (These measurements were not included in the Results.) Rotation toward the nerve causes the nerve to lie in the resulting space between the device and the TCL. In both groups A and B, flexor tendons were found dorsal or ulnar to the device but were never found between the device and the median nerve or between the device and the TCL. A separate fasciat layer was seen directly palmar to the TCL in seven of nine specimens but could not be easily separated from it. The layer was derived primarily from thenar muscle fascia, but it also received contributions from the hypothenar muscle fascia and the dorsal fascia of the palmaris brevis overlying Guyon s canal. The thickness of this layer decreased as it went from the radial side to the ulnar side, measuring 3.5 to 0.5 mm in thickness (Fig. 7, A and B). It was seen more clearly in the distal carpal canal sections in specimens with more developed thenar musculature. Sagittal sections (Table II). In group A (RF axis)

6 Vol. 18A, No. 3 May 1993 Endoscopic carpal tunnel device 447 Fig. 7. A, Transverse section showing separate layer arising from thenar muscle fascia overlying TCL. This layer is being held by forceps. M, Blackened median nerve. (Original magnification x 6.3.). B, Diagram of transverse section taken from distal half of carpal tunnel. The separate fascial layer seen directly palmar to the TCL is derived primarily from thenar muscle fascia but also receives contributions from hypothenar muscle fascia and dorsal fascia of the palmaris brevis. the distance to the SPA averaged 4.8 mm distal to the distal edge of the TCL. The proximal extent of the fat pad was 2.8 mm proximal to the distal edge of the TCL (Fig. 8, A). In group B (LRI axis), the SPA averaged 5.5 distal to the distal edge of the TCL. The proximal extent of the fat pad was located almost immediately at the distal margin of the TCL, averaging only 0.8 mm proximal (Fig. 8, B). In this axis the distal edge of the TCL was at times difficult to determine because of its junction with the palmar fascia. Comparisons between the two axes are noted in Fig. 8, C and D; the differences

7 448 Rotman and Manske The Journal of HAND SURGERY Distance (ram) t) t Distance from TCL to SPA Distance from Fat Pad to TCL RF Axis LRI Distance 4.0 o D (mm) Axis Fig. 8. A, Sagittal section along RF axis showing distal edge of TCL (black arrow), SPA (asterisk), and proximal extent of the pad (white arrow). The distance between the distal edge of the TCL and the SPA is 6 mm in this specimen; the fat pad begins 3 mm proximal to the distal edge of the ligament. B, Sagittal section along LR1 axis showing proximal extent of fat pad (white arrow) originating near distal edge of TCL. The SPA (asterisk) is noted on the left. (Original magnification 10.) C, Distances from distal edge of TCL to SPA in RF and LRI axes are compared. The difference is not statistically significant. D, Distances from proximal extent of fat pad to distal edge of TCL in RF and LRI axes are compared. The difference is statistically significant between the two (p 0.02). LRI related to the proximal extent of the fat pad are statistically significant between the two axes. Composite sections. At the junction between the transverse fibers of the TCL and the overlying structures, the palmar fascia was found to overlie the TCL only along the LF axis and the LRI axis, but not along the RF axis. The overlying fascial layer derived from the thenar and hypothenar muscles (described above) could not be differentiated from the transverse collagen TCL fibers in either of the axes. In the anterior half of the ligament, TCL collagen fibers were interspersed with thenar and hypothenar muscle fascicles. Adjacent structures found directly superficial to the TCL in the RF axis consisted of subcutaneous tissue mixed with fibrous connective tissue and small blood vessels and small nerves. Discussion The results of this study underscore the importance of understanding the anatomic relationships between an endoscopic device and.surrounding soft tissue structures when performing endoscopi c carpal tunnel release. Although the results presented in this study apply directly to the use of the Inside Job device, they may be applicable to the use of other endoscopic devices 6 8 inasmuch as anatomic relationships are similar.

8 Vol. 18A, No. 3 May 1993 Endoscopic carpal tunnel device 449 Median nerve and flexor tendons. The recommended surgical approach for both endoscopic and open release of the carpal tunnel is in the line of the RF axis.4, 9, With respect to open release of the carpal tunnel, this axis avoids damage to the palmar cutaneous branches of both the median and ulnar nerves During endoscopic carpal tunnel release, this recommended axis presumably avoids placement of the endoscope blade directly beneath the median nerve. The results emphasize the importance of placing the device in the RF axis and the jeopardy of straying into the LRI axis. In this small series, no ulnar branches of the median nerve as described by Lanz ~5 in 1% of carpal tunnel explorations were observed. If this accessory branch or any other anatomic structure is found superficial to or passing through the transverse carpal ligament, it could be injured by endoscopic carpal tunnel release. Fortunately, most anatomic variations of the median nerve or its branches are found radial or directly palmar to the nerve and are not likely to be injured. Injuries to either the main trunk of the median nerve or its common digital branches have been reported with the Inside Job 5,10 and the two-portal technique described by Chow. 9 It is not clear how these injuries were caused. We found that the device easily rotates toward the nerve during insertion, inasmuch as it tends to conform to the curve of the lateral border of the hamate. The surgeon has to be careful to keep the device snugly abutted against the undersurface of the ligament and not rotate it toward the nerve. There is also a tendency to lever the device against the hook of the hamate and point it toward the LRI axis. The flexor tendons were found to be either on the ulnar side or dorsal to the Inside Job housing in all specimens. There was no instance in which a flexor tendon was found between the device and the nerve or between the device and the TCL. With dissection of the ulnar bursa off the overlying transverse carpal ligament, the flexor tendons are pushed away from the device and are protected from injury. Dissection of the ulnar bursa allows visualization of the transverse fibers of the TCL. It is important that these fibers be clearly identified and visualized at the time of ligament incision. Any anomalous structures along the undersurface of the TCL (e.g., palmaris profundus) 17 that prevent visualization of the transverse fibers should caution the surgeon against proceeding with endoscopic release. SPA and fat pad. The TCL begins proximally as a continuation of the antebrachial fascia and then ends distally by blending with the fibers of the midpalmar fascia. The ligament varies in length from 26 to 34 mm. 18 In this study the distal edge of the TCL was more clearly defined along the RF axis by its junction with a layer of loose, cellular connective tissue mixed with fat. Kaplan and Milford ~ described this fat pad as the proximal aspect of the fatty tissue that continues distally and invests the superficial palmar arterial arch, the common digital nerves, and the synovial bursa of the flexor tendons. Although Kaplan and Milford do not indicate whether this fat envelops the distal border of the TCL, our studies clearly show that it does so along the RF axis. This fat pad consistently extended 2 to 3.5 mm proximal to the distal edge of the TCL in the RF axis. Failure to incise the ligament beyond the proximal extent of this fat would result in an incomplete release of the distal TCL fibers. In this study, the SPA was approximately 5 mm distal to the TCL. Okutsu et al. 8 indicated that the SPA was 10 to 15 mm distal to the distal margin of the TCL but gave no details as to how the measurements were made. With careful endoscopic release along the RF axis, the SPA is not likely to be in jeopardy. Superficial fascial structures. The improved shortterm clinical results noted with endoscopi carpal tunnel release, as compared with open carpal tunnel release, are attributed in part to preservation of the continuity of the more superficial fascial structures palmar to the TCL; this continuity is thought to achieve stronger postoperative grip strength than standard carpal tunnel release (personal communication from J.M. Agee and from J.C.Y. Chow). Clinically, during endoscopic release, Agee and Chow have noted that the incised margins of the TCL will separate but will leave transverse collagen fibers intact palmar to it. The identity of these fibers has not previously been clarified. We do not believe they represent fibers of the palmar fascia or palmaris brevis. In composite histologic sections, longitudinal palmar fascial fibers were found only along the LF and LRI axes; there was no evidence ofpalmar fascia overlying the TCL in the RF axis. Furthermore, although transverse fibers of palmar fascia have been described, they are located distal to the TCL.19.2o. A separate fascial structure was noted immediately palmar to the TCL arising from the thenar musculature and joining ulnarly with the hypothenar muscle fascia and the dorsal or deep fascia of the palmaris brevis that overlies Guyon s canal. This layer could not be mechanically separated from the TCL; nor could it be differentiated histologically from TCL collagen fibers. We propose that the fascial layer that we have described represents the intact transverse collagen fibers noted by

9 450 Rotman and Manske The Journal of HAND SURGERY Agee and Chow after endoscopic carpal tunnel release. Both thenar and hypothenar muscles take origin partially from the TCL. ~ These muscle fascicles were found interspersed within the transverse collagen fibers in the anterior half of the TCL. These muscle fascicles, if encountered during endoscopic release, should not be confused with muscle fascicles originating from the palmaris brevis muscle, which would be found palmar to 2~ the transverse carpal ligament. The purpose of this study is not to advocate the use of an endoscopic device in carpal tunnel release. However, if the surgeon chooses to use such a device, knowledge of the anatomic relationships with regard to the position of the endoscopic device during carpal tunnel release is of critical importance. Awareness of the anatomic relationships presented in this study may enable the surgeon to avoid potential complications. The surgeon is advised that it is important to (1) dissect the ulnar bursa away from the overlying transverse carpal ligament, (2) position the device along the ring finger axis, (3) be aware that the distal margin of the TCL extends approximately 2.5 to 3.0 mm into the fat pad, and (4) be aware that the superficial palmar arch 5 mm distal to the TCL within this investing fat pad. Inside Job blade housing assemblies were provided by the Orthopedic Products Division, 3M Health Care, St. Paul, Minn. REFERENCES I. Kaplan EB, Milford LW. The retinacular system of the hand. In: Spinner M, ed. 3rd ed. Kaplan s functional and surgical anatomy of the hand. Philadelphia: JB Lippincott, 1984: Robbins H. Anatomical study of the median nerve in the carpal tunnel and etiologies of the carpal-tunnel syndrome. J Bone Joint Surg 1963;45A: Tanzer RC. The carpal-tunnel syndrome: a clinical and anatomical study. J Bone Joint Surg 1959;41A: The Agee surgical technique and user s guide. Agee Inside Job (TM) carpal tunnel release system. St. Paul: Orthopedic Products Division, 3M Health Care, Agee JM, Tortosa R, Barry D, Peimer CA. Endoscopic release of the carpal tunnel: a randomized prospective multicenter study. J HAND SUr~G 1992;17A: Chow JCY. Endoscopic release of the carpal ligament: a new technique for carpal tunnel syndrome. Arthroscopy 1989;5: Chow JCY. Endoscopic release of the carpal ligament for carpal tunnel syndrome: 22 month clinical result. Arthroscopy 1990;6: Okutsu I, Ninomiya S, Takatori Y, Ugawa Y. Endoscopic management of carpal tunnel syndrome. Arthroscopy 1989;5: Resnick CT, Miller BW. Endoscopic carpal tunnel release using the subligamentous two-portal technique. Contemp Orthop 1991;22: FarrAC. Scope carpal tunnel release nerve risk outweighs benefit. Orthop Today 1992; 12: I. 11. Sheehan DC, Hrapchak BB. Theory and practice of histotechnology. 2nd ed. St. Louis: CV Mosby, DeLeon AL, Rojkind M. A simple micro method for collagen and total protein determination in formalin-fixed paraffin-embedded section. J Histochem Cytochem 1985;33: Engber WD, Gmeiner JG. Palmar cutaneous branch of the ulnar nerve. J HAND SURG 1980;5: Jones JA, Burton RI. Carpal tunnel syndrome: treatment, pitfalls, and failures. Surg Rounds Orthop 1988;2: Lanz U. Anatomical variations of the median nerve in the carpal tunnel. J HAND SURG 1977;2: Taleisnik J. The palmar cutaneous branch of the median nerve and the approach to the carpal tunnel: an anatomical study. J Bone Joint Surg 1973;55A: Lahey MD, Aulicino PL. Anomalous muscles associated with compression neuropathies. Orthop Rev 1986; 15: Johnson RK, Shrewsbury MM. Anatomical course of the thenar branch of the median nerve--usually in a separate tunnel through the transverse carpal ligament. J Bone Joint Surg 1970;52A: Caughell KA, McFarlane RM, McGrouther DA, Martin AH. Developmental anatomy of the palmar aponeurosis and its relationship to the palmaris longus tendon. J HAteD SURG 1988;13A: Ritter MA. The anatomy and function of the palmar fascia. Hand 1973;5: Shrewsbury M, Johnson RK, Ousterhout DK. The palmaris brevis: a reconsideration of its anatomy and possible function. J Bone Joint Surg 1972;54A:344-8.