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Assessment of nerve graft donor sites used for reconstruction of traumatic digital nerve defects
Assessment of Nerve Graft Donor Sites Used for Reconstruction of Traumatic Digital Nerve Defects James P. Higgins, MD, Stephen Fisher, MD, Joseph M. Serletti, MD, Greg S. Orlando, MD, Rochester, NY Several donor nerve graft sites commonly are used when repairing segmental defects in sensory nerves distal to the wrist. The cross-sectional area and number of fascicles of donor nerves and specific digital nerve segments were investigated to provide guidelines for selection of nerve graft harvest sites according to defects encountered. Nerve segments were harvested from 10 fresh cadavers (20 upper extremities). Five sites of nerve graft were harvested: lateral and medial antebrachial cutaneous nerves (LABCN, MABCN), posterior and anterior interosseous nerves (PIN, AIN), and sural nerves. Four sites of typical segmental nerve defects were harvested in a zone protocol: common digital nerve (zone 4), proper digital nerve (zone 3), digital nerve distal to main dorsal branch at the metacarpophalangeal joint (zone 2), and digital nerve distal to trifurcation at fingertip (zone 1). Sural nerve is the most anatomically similar nerve graft for defects in zone 4 by cross-sectional area and number of fascicles. Lateral antebrachial cutaneous nerve is most appropriate for zone 2 and 3 injuries by both criteria. Fingertip grafts for zone 1 injuries displayed cross-sectional area similarity to PIN, AIN, and MABCN. With regard to number of fascicles, zone 1 digital nerves are most similar to LABCN donors. (J Hand Surg 2002;27A:286 –292. Copyright © 2002 by the American Society for Surgery of the Hand.) Key words: Nerve grafting, digital nerve.
Reconstruction of the traumatized upper extremity often requires the repair of segmental defects in sensory nerves distal to the wrist. Inability to achieve tension-free primary repair necessitates use of interpositional conduit material. Although research on autogenous vascular and muscle conduits and nonFrom the Division of Plastic Surgery, Department of Orthopaedic Surgery, Center for Musculoskeletal Research, University of Rochester, Rochester, NY. Received for publication April 27, 2001; accepted in revised form November 16, 2001. 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 P. Higgins, MD, 4816 Clairelee Dr, Owings Mills, MD 21117. Copyright © 2002 by the American Society for Surgery of the Hand 0363-5023/02/27A02-0344$35.00/0 doi:10.1053/jhsu.2002.31154
286 The Journal of Hand Surgery
autogenous biologic and synthetic conduits continues, conventional autogenous nerve grafting remains a widely used technique for repair of segmental nerve defects. Several donor sites commonly are used for nerve graft harvest. Criteria for selection of donor site for nerve harvest have not been standardized. Many factors are assessed in selection of the site for nerve graft harvest, but primarily the surgeon seeks a nerve of similar caliber to the injured nerve. Distal to the wrist the caliber and fascicular density of digital nerves along the pathway from common digital nerve to fingertip can vary greatly. The purpose of this study is to provide guidelines for nerve graft selection by detecting similarities between common donor nerves and specific digital nerve segments with regard to the cross-sectional area and number of nerve fascicles.
The Journal of Hand Surgery / Vol. 27A No. 2 March 2002 287
Figure 1. Anatomic zone system of digital nerve from wrist to fingertip. Zone 1, distal to trifurcation in distal phalanx; zone 2, major dorsal sensory branch takeoff at MCP to trifurcation; zone 3, common digital nerve bifurcation to dorsal branch at MCP; zone 4, wrist to common digital nerve bifurcation.
Materials and Methods An anatomic zoning system was created to categorize typically encountered digital nerve defects of the hand (Fig. 1). Four zones were established in relation to major anatomic landmarks that marked caliber changes in the digital nerve pathway. In the fingertip the digital nerve distal to the trifurcation at the distal interphalangeal joint was labeled zone 1. Proximal to the trifurcation and distal to the takeoff of the major dorsal branch at the metacarpophalangeal (MCP) joint was identified as zone 2. Proximal to the MCP dorsal branch and distal to the common digital nerve bifurcation was classified as zone 3. Proximal to the common digital nerve bifurcation and distal to the wrist was labeled zone 4. Five common donor sites for nerve grafting in traumatic hand injuries were selected for study: the sural nerve, medial antebrachial cutaneous nerve (MABCN), lateral antebrachial cutaneous nerve (LABCN), posterior interosseous nerve (PIN), and anterior interosseous nerve (AIN). Nine fresh cadaver specimens were studied. One nerve sample from each of the digital nerve zones was harvested from each hand. To standardize the data, these were sampled solely from the radial digital nerve to the middle finger. Each of the donor
nerves also was collected from each side of each cadaver. Eighteen specimens were collected for each of the 4 digital nerve zones and each of the 5 donor nerve sites. One cadaver had evidence of wrist trauma requiring exclusion of the digital and interosseous nerve samples. Three other samples were excluded because of damage or artifact during the staining and sectioning process. The same 2 individuals working together harvested all nerves (J.P.H., S.F.). Care was taken to handle the nerves atraumatically without any time limit on harvest. Guidelines were created to standardize the harvest of all nerve samples. In zone 1 the largest of the nerves distal to the trifurcation was selected. In zone 2 the specimen was harvested immediately distal to the takeoff of the MCP dorsal branch. In zone 3 all specimens similarly were harvested immediately distal to the common digital nerve bifurcation. In zone 4 a segment immediately proximal to the bifurcation was selected. When harvesting the donor nerve samples, we established guidelines to mimic the location of routine clinical harvest. The sural nerve harvest site was selected by measuring proximal to the lateral malleolus one fifth of the distance from the lateral malleolus to the fibular head. The anterior and posterior interosseous nerves were harvested at the midpoint of the pronator quadratus muscle belly; both were harvested by way of a dorsal approach. The lateral and medial antebrachial nerves were harvested at the junction between the proximal and middle thirds of the distance between Lister’s tubercle and the antecubital fossa crease. Although previous investigators have described harvesting the medial antebrachial cutaneous nerve above or below the elbow,1 we decided to standardize our harvest below the elbow as described in other reports and an outcome study on the use of this nerve for digital nerve grafting.2,3 If 2 or more branches were encountered, the largest was harvested. Harvested nerve specimens were placed immediately in a 10% buffered formalin solution. Specimens were mounted in paraffin blocks, and histologic sections of 5-m thickness were prepared in a crosssectional plane and stained with hematoxylin-eosin (Figs. 2, 3). Samples were examined under light microscopy, and numbers of fascicles were recorded. Images were digitally captured under ⫻4 magnification with Snappy Video Snapshot software (Play Inc, Rancho Cordova, CA). These images were analyzed with Scion Image Beta 4.02 software (Scion Corp,
288 Higgins et al / Assessment of Nerve Graft Donor Sites
Figure 2. Zone 4 digital nerve. (Hematoxylin-eosin stain; original magnification ⫻2.)
Frederick, MD) to determine the cross-sectional area of the nerves in square millimeters. The data were analyzed with Microsoft Excel software (Microsoft, Redmond, WA) to determine the mean cross-sectional area and mean number of fas-
cicles. SD also was calculated. The cross-sectional area and number of fascicles data for each donor nerve site were compared with cross-sectional area and number of fascicles data for each digital nerve zone site. These data were subjected to a 2-tailed
Figure 3. Sural nerve. (Hematoxylin-eosin stain; original magnification ⫻2.)
The Journal of Hand Surgery / Vol. 27A No. 2 March 2002 289
Figure 5. Mean number of fascicles of all sample sites with SD. Figure 4. Mean cross-sectional area of all sample sites with SD.
t-test. A p value ⬍ .05 showed a significant difference between the cross-sectional area (or number of fascicles) of the nerve graft site and the digital nerve site with a 95% confidence interval. A p value of ⬎.05 showed that the nerve samples were not statistically different in caliber or number of fascicles and were an appropriate size or caliber match for nerve grafting.
Results Cross-sectional Area The cross-sectional area mean values and SDs for all nerve specimens are shown in Figure 4. p values comparing digital nerve zones with donor nerves are given in Table 1. For defects in zone 1, the fingertip pulp, AIN, PIN, and MABCN are similar in crosssectional area. For zones 2 and 3 from the trifurcation at the fingertip to the bifurcation of the common
digital nerve, LABCN showed caliber similarity. For zone 4 in the common digital nerve distal to the wrist and proximal to the bifurcation, sural nerve grafts were the closest in size match but still were significantly smaller than the common digital nerve samples (p ⬍ .05).
Number of Fascicles The number of nerve fascicles (mean values and SDs) for all nerve specimens are shown in Figure 5. p values comparing digital nerve zones with donor nerves are given in Table 2. For defects in zone 1, the fingertip pulp, LABCN showed similar numbers of fascicles. For zone 2 from the trifurcation at the fingertip to the dorsal branch takeoff at the MCP joint, LABCN was the closest match but displayed significantly fewer fascicles than the digital nerve (p ⬍ .05). In zone 3 proximal to the MCP dorsal branch takeoff and distal to the common digital nerve bifurcation, LABCN showed similar numbers of fascicles. For zone 4 in the common digital nerve distal to the
Table 1. p Values of t-Test Comparison of Digital Nerve Zones and Donor Nerves: Cross-sectional Area Zone 1 AIN PIN LABCN MABCN Sural nerve
.2858* .1582* .0001 .6856* 1.5750 ⫻ 10⫺10
Zone 2
Zone 3 ⫺10
6.7428 ⫻ 10 5.5477 ⫻ 10⫺7 .3149* 5.2823 ⫻ 10⫺5 7.8274 ⫻ 10⫺9
Zone 4 ⫺12
3.2720 ⫻ 10 7.6710 ⫻ 10⫺10 .1050* 1.7936 ⫻ 10⫺8 1.2051 ⫻ 10⫺6
*No statistically significant difference in cross-sectional area between donor and recipient nerves.
2.1912 ⫻ 10⫺13 3.7655 ⫻ 10⫺13 7.5771 ⫻ 10⫺12 1.0556 ⫻ 10⫺14 .0071
290 Higgins et al / Assessment of Nerve Graft Donor Sites
Table 2. p Values of t-Test Comparison of Digital Nerve Zones and Donor Nerves: Number of Fascicles Zone 1 AIN PIN LABCN MABCN Sural nerve
.0078 3.31 ⫻ 10⫺6 .8093* .0226 .0003
Zone 2
Zone 3
.0001 3.79 ⫻ 10⫺8 .0294 .0002 .0201
.0002 2.26 ⫻ 10⫺11 .4250* .0018 5.08 ⫻ 10⫺5
Zone 4 1.32 ⫻ 10⫺11 1.74 ⫻ 10⫺15 3.78 ⫻ 10⫺11 8.59 ⫻ 10⫺12 .0103
*No statistically significant difference in number of fascicles between donor and recipient nerves.
wrist and proximal to the bifurcation, sural nerve grafts were the closest match but still had significantly fewer fascicles than the common digital nerve samples (p ⬍ .05).
Discussion Primary tensionless repair is the desired treatment for traumatically transected digital nerves. In the setting of segmental nerve loss resulting from avulsion, crushed and debrided margins, or contraction and scarring in delayed repair, interposition conduits are required to obtain a tension-free coaptation. The pursuit of alternative nerve conduits has been the subject of research exploring autogenous vascular4 and muscle5 and nonautogenous biologic6 and synthetic conduits.7 Despite these experimental data, autogenous nerve grafts remain the most common conduits used for segmental defects. Much of the literature on the use of autogenous nerve grafting addresses surgical technique and functional outcomes. Standard protocols for the selection of nerve donor sites for specific anatomic defects have not been developed. The surgeon must consider many variables when selecting donor nerves, including ease of harvest, visibility of donor site scar, magnitude of donor site sensory deficit, length of available nerve, and caliber of the donor nerve. Harvestable length of the various donor nerves is well described in peripheral nerve surgery literature.8,9 Each available donor site offers the surgeon specific qualitative advantages and disadvantages. Anterior interosseous nerve and PIN, although more difficult to harvest, result in negligible donor sensory deficits. The dorsal forearm scar is minimal but conspicuous. Medial antebrachial cutaneous nerve and LABCN are harvested easily and leave less conspicuous scars, but the donor sensory deficit is more notable. Use of the anterior branch of MABCN avoids sensory losses to the elbow and posterior proximal fore-
arm, the crucial dermatome supplied by the posterior branch of the same nerve.10 Harvest of LABCN leaves a sensory deficit in the radial aspect of the volar forearm, but the dermatome overlap with the sensory branch of the radial nerve in 75% of the population minimizes this deficit.11 The sural nerve provides the greatest available length of 30 to 40 cm. The donor site scar can be unsightly, particularly if a longitudinal approach is used rather than the stepladder transverse open approach or an endoscopic harvest. The harvest is not technically difficult, particularly when only a short length is required for nerve grafting in the hand. Loss of the sural nerve results in a sensory deficit on the lateral aspect of the foot that diminishes over time.5 In contrast to these qualitative considerations to selecting nerve graft harvest sites, guidelines for caliber matching of donor nerves to specific defects have not been established, perhaps because no studies have examined the impact of nerve graft caliber matching on functional outcome. The importance of selecting nerve grafts of similar caliber to the defects remains intuitive. A caliber-matched nerve interface leads to more accurate technical repair. Subsequently the interface requires minimal sutures and perhaps results in less scarring to achieve alignment. Whether fewer sutures and less scarring achieve superior sensory recovery requires future investigation. Cable grafting is a method of achieving reasonable caliber matching between segmental nerve defects and interpositional grafts. The disadvantage of cable grafting often cited is the large percentage of the graft surface area that is not in contact with the recipient bed. Revascularization of these free grafts is through spontaneous anastomoses between vessels in the recipient bed and the vessels on the graft’s surface. It is believed that the central portions of cable grafts are unlikely to be revascularized and the endoneurial pattern of the Schwann cells is more
The Journal of Hand Surgery / Vol. 27A No. 2 March 2002 291
Table 3. Appropriate Donor Nerve Graft Sites by Anatomic Zone: Cross-sectional Area and Fascicle Number Zone 1
Zone 2
Zone 3
Zone 4
Cross-sectional area
AIN* PIN* MABCN*
LABCN*
LABCN*
Sural nerve
Number of fascicles
LABCN*
LABCN
LABCN*
Sural nerve
*Bold lettering signifies statistical significance.
likely to become fibrotic.9 Cable grafting is technically more difficult, requiring more sutures for alignment and potentially resulting in greater scarring at the graft interface. The choice of cable grafting rather than using a graft of appropriate diameter requires greater length of donor nerve, potentially resulting in greater dissection and larger harvest scars. Although the technique of cable grafting may be used to avoid other comorbidities of specific donor sites (ie, sensory loss at the lateral aspect of the foot), we believe that conventional nerve grafting with well caliber matched donor nerves most likely results in favorable outcome and we reserve cable grafting for occasions when appropriate size single grafts are not available. We used an anatomic zoning system to detect precise changes in caliber and number of fascicles along the pathway of the digital nerve from the wrist to fingertip (Table 3). Samples within this zoning system were matched accurately with donor nerves. In this manner guidelines were developed for donor nerve selection for each zone according to the same 2 criteria of caliber and number of fascicles. When grafting defects in zone 4 (the common digital nerve proximal to the bifurcation), the sural nerve is the most appropriate choice of donor nerve. Although displaying considerably fewer fascicles and smaller cross-sectional area than the zone 4 digital nerve, it is the closest available match among the selected donor nerves in both criteria examined. Lateral antebrachial cutaneous nerve is the best match for deficits in zones 2 and 3 (from the fingertip trifurcation to the common digital nerve bifurcation). There is no major difference in the caliber match between LABCN and the digital nerve in zones 2 and 3. The fascicle numbers also are equivalent for zone 3, and although LABCN has markedly fewer fascicles than zone 2 nerves, it is the closest match among the donors studied. Lateral antebrachial cutaneous nerve can be har-
vested with ease, and its resultant sensory deficit overlaps with the dermatome of the sensory branch of the radial nerve.8 In the fingertip distal to the digital nerve trifurcation (zone 1), AIN, PIN, and MABCN all are appropriate choices for caliber-matched grafts. Although these donor nerves display caliber similarity to the zone 1 defects, LACBN is the only similar donor nerve when number of fascicles is assessed. This is the only zone where the 2 comparison criteria produced different results. The relative importance of these 2 nerve graft criteria (cross-sectional area and number of fascicles) to sensory outcomes has not been studied. This is an area of future investigation to provide guidance in selection of donor nerve grafts in this region. In our institution zone 1 defects are repaired by caliber matching as the criteria for donor nerve selection for 2 reasons: (1) We perform standard epineurial repair without consideration for anatomic alignment of fascicles in that region, and (2) grafting in this zone is technically demanding and can be simplified by use of a well caliber matched graft. Among the caliber-matched options, we commonly select PIN as a zone 1 conduit for its ease of harvest and minimal donor site sensory deficit.
References 1. Masear VR, Meyer RD, Pichora DR. Surgical anatomy of the medial antebrachial cutaneous nerve. J Hand Surg 1989; 14A(2 pt 1):267–271. 2. Brushart TM. Nerve repair and grafting. In: Green DP, Hotchkiss RN, Pederson WC, eds. Operative hand surgery. 4th ed. Philadelphia: Churchill Livingstone, 1999:1381– 1403. 3. Nunley JA, Ugino MR, Goldner RD, Regan N, Urbaniak JR. Use of the anterior branch of the medial antebrachial cutaneous nerve as a graft of defects of the digital nerve. J Bone Joint Surg 1989;71A:563–567. 4. Chui DTW, Strauch B. A prospective clinical evaluation of
292 Higgins et al / Assessment of Nerve Graft Donor Sites autogenous vein grafts used as a nerve conduit for distal sensory nerve defects of 3 cm or less. Plast Reconstr Surg 1990;86:928 –934. 5. Norris RW, Glasby MA, Guttuso JM, Bowden REM. Peripheral nerve repair in humans using muscle autografts. A new technique. J Bone Joint Surg 1988;70B: 530 –533. 6. Mackinnon SE, Dellon AL, Hudson AR, Hunter DA. Nerve regeneration through a pseudosynovial sheath in a primate model. Plast Reconstr Surg 1985;75:833– 839. 7. Mackinnon SE, Dellon AL. Clinical nerve reconstruction
with a bioabsorbable polyglycolic acid tube. Plast Reconstr Surg 1990;85:419 – 424. 8. Mackinnon SE, Dellon AL. Surgery of the peripheral nerve. New York: Thieme Medical, 1988:102–107. 9. Diao E, Vannuyen T. Techniques for primary nerve repair. Hand Clin 2000;16:53– 66. 10. Race CM, Saldana MJ. Anatomic course of the medial cutaneous nerves of the arm. J Hand Surg 1991;16A:48 –52. 11. Mackinnon SE, Dellon AL. The overlap pattern of the lateral antebrachial cutaneous nerve and the superficial branch of the radial nerve. J Hand Surg 1985;10A:522–526.
Reconstruction of the traumatized upper extremity often requires the repair of segmental defects in sensory nerves distal to the wrist. Inability to achieve tension-free primary repair necessitates use of interpositional conduit material. Although research on autogenous vascular and muscle conduits and nonFrom the Division of Plastic Surgery, Department of Orthopaedic Surgery, Center for Musculoskeletal Research, University of Rochester, Rochester, NY. Received for publication April 27, 2001; accepted in revised form November 16, 2001. 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 P. Higgins, MD, 4816 Clairelee Dr, Owings Mills, MD 21117. Copyright © 2002 by the American Society for Surgery of the Hand 0363-5023/02/27A02-0344$35.00/0 doi:10.1053/jhsu.2002.31154
286 The Journal of Hand Surgery
autogenous biologic and synthetic conduits continues, conventional autogenous nerve grafting remains a widely used technique for repair of segmental nerve defects. Several donor sites commonly are used for nerve graft harvest. Criteria for selection of donor site for nerve harvest have not been standardized. Many factors are assessed in selection of the site for nerve graft harvest, but primarily the surgeon seeks a nerve of similar caliber to the injured nerve. Distal to the wrist the caliber and fascicular density of digital nerves along the pathway from common digital nerve to fingertip can vary greatly. The purpose of this study is to provide guidelines for nerve graft selection by detecting similarities between common donor nerves and specific digital nerve segments with regard to the cross-sectional area and number of nerve fascicles.
The Journal of Hand Surgery / Vol. 27A No. 2 March 2002 287
Figure 1. Anatomic zone system of digital nerve from wrist to fingertip. Zone 1, distal to trifurcation in distal phalanx; zone 2, major dorsal sensory branch takeoff at MCP to trifurcation; zone 3, common digital nerve bifurcation to dorsal branch at MCP; zone 4, wrist to common digital nerve bifurcation.
Materials and Methods An anatomic zoning system was created to categorize typically encountered digital nerve defects of the hand (Fig. 1). Four zones were established in relation to major anatomic landmarks that marked caliber changes in the digital nerve pathway. In the fingertip the digital nerve distal to the trifurcation at the distal interphalangeal joint was labeled zone 1. Proximal to the trifurcation and distal to the takeoff of the major dorsal branch at the metacarpophalangeal (MCP) joint was identified as zone 2. Proximal to the MCP dorsal branch and distal to the common digital nerve bifurcation was classified as zone 3. Proximal to the common digital nerve bifurcation and distal to the wrist was labeled zone 4. Five common donor sites for nerve grafting in traumatic hand injuries were selected for study: the sural nerve, medial antebrachial cutaneous nerve (MABCN), lateral antebrachial cutaneous nerve (LABCN), posterior interosseous nerve (PIN), and anterior interosseous nerve (AIN). Nine fresh cadaver specimens were studied. One nerve sample from each of the digital nerve zones was harvested from each hand. To standardize the data, these were sampled solely from the radial digital nerve to the middle finger. Each of the donor
nerves also was collected from each side of each cadaver. Eighteen specimens were collected for each of the 4 digital nerve zones and each of the 5 donor nerve sites. One cadaver had evidence of wrist trauma requiring exclusion of the digital and interosseous nerve samples. Three other samples were excluded because of damage or artifact during the staining and sectioning process. The same 2 individuals working together harvested all nerves (J.P.H., S.F.). Care was taken to handle the nerves atraumatically without any time limit on harvest. Guidelines were created to standardize the harvest of all nerve samples. In zone 1 the largest of the nerves distal to the trifurcation was selected. In zone 2 the specimen was harvested immediately distal to the takeoff of the MCP dorsal branch. In zone 3 all specimens similarly were harvested immediately distal to the common digital nerve bifurcation. In zone 4 a segment immediately proximal to the bifurcation was selected. When harvesting the donor nerve samples, we established guidelines to mimic the location of routine clinical harvest. The sural nerve harvest site was selected by measuring proximal to the lateral malleolus one fifth of the distance from the lateral malleolus to the fibular head. The anterior and posterior interosseous nerves were harvested at the midpoint of the pronator quadratus muscle belly; both were harvested by way of a dorsal approach. The lateral and medial antebrachial nerves were harvested at the junction between the proximal and middle thirds of the distance between Lister’s tubercle and the antecubital fossa crease. Although previous investigators have described harvesting the medial antebrachial cutaneous nerve above or below the elbow,1 we decided to standardize our harvest below the elbow as described in other reports and an outcome study on the use of this nerve for digital nerve grafting.2,3 If 2 or more branches were encountered, the largest was harvested. Harvested nerve specimens were placed immediately in a 10% buffered formalin solution. Specimens were mounted in paraffin blocks, and histologic sections of 5-m thickness were prepared in a crosssectional plane and stained with hematoxylin-eosin (Figs. 2, 3). Samples were examined under light microscopy, and numbers of fascicles were recorded. Images were digitally captured under ⫻4 magnification with Snappy Video Snapshot software (Play Inc, Rancho Cordova, CA). These images were analyzed with Scion Image Beta 4.02 software (Scion Corp,
288 Higgins et al / Assessment of Nerve Graft Donor Sites
Figure 2. Zone 4 digital nerve. (Hematoxylin-eosin stain; original magnification ⫻2.)
Frederick, MD) to determine the cross-sectional area of the nerves in square millimeters. The data were analyzed with Microsoft Excel software (Microsoft, Redmond, WA) to determine the mean cross-sectional area and mean number of fas-
cicles. SD also was calculated. The cross-sectional area and number of fascicles data for each donor nerve site were compared with cross-sectional area and number of fascicles data for each digital nerve zone site. These data were subjected to a 2-tailed
Figure 3. Sural nerve. (Hematoxylin-eosin stain; original magnification ⫻2.)
The Journal of Hand Surgery / Vol. 27A No. 2 March 2002 289
Figure 5. Mean number of fascicles of all sample sites with SD. Figure 4. Mean cross-sectional area of all sample sites with SD.
t-test. A p value ⬍ .05 showed a significant difference between the cross-sectional area (or number of fascicles) of the nerve graft site and the digital nerve site with a 95% confidence interval. A p value of ⬎.05 showed that the nerve samples were not statistically different in caliber or number of fascicles and were an appropriate size or caliber match for nerve grafting.
Results Cross-sectional Area The cross-sectional area mean values and SDs for all nerve specimens are shown in Figure 4. p values comparing digital nerve zones with donor nerves are given in Table 1. For defects in zone 1, the fingertip pulp, AIN, PIN, and MABCN are similar in crosssectional area. For zones 2 and 3 from the trifurcation at the fingertip to the bifurcation of the common
digital nerve, LABCN showed caliber similarity. For zone 4 in the common digital nerve distal to the wrist and proximal to the bifurcation, sural nerve grafts were the closest in size match but still were significantly smaller than the common digital nerve samples (p ⬍ .05).
Number of Fascicles The number of nerve fascicles (mean values and SDs) for all nerve specimens are shown in Figure 5. p values comparing digital nerve zones with donor nerves are given in Table 2. For defects in zone 1, the fingertip pulp, LABCN showed similar numbers of fascicles. For zone 2 from the trifurcation at the fingertip to the dorsal branch takeoff at the MCP joint, LABCN was the closest match but displayed significantly fewer fascicles than the digital nerve (p ⬍ .05). In zone 3 proximal to the MCP dorsal branch takeoff and distal to the common digital nerve bifurcation, LABCN showed similar numbers of fascicles. For zone 4 in the common digital nerve distal to the
Table 1. p Values of t-Test Comparison of Digital Nerve Zones and Donor Nerves: Cross-sectional Area Zone 1 AIN PIN LABCN MABCN Sural nerve
.2858* .1582* .0001 .6856* 1.5750 ⫻ 10⫺10
Zone 2
Zone 3 ⫺10
6.7428 ⫻ 10 5.5477 ⫻ 10⫺7 .3149* 5.2823 ⫻ 10⫺5 7.8274 ⫻ 10⫺9
Zone 4 ⫺12
3.2720 ⫻ 10 7.6710 ⫻ 10⫺10 .1050* 1.7936 ⫻ 10⫺8 1.2051 ⫻ 10⫺6
*No statistically significant difference in cross-sectional area between donor and recipient nerves.
2.1912 ⫻ 10⫺13 3.7655 ⫻ 10⫺13 7.5771 ⫻ 10⫺12 1.0556 ⫻ 10⫺14 .0071
290 Higgins et al / Assessment of Nerve Graft Donor Sites
Table 2. p Values of t-Test Comparison of Digital Nerve Zones and Donor Nerves: Number of Fascicles Zone 1 AIN PIN LABCN MABCN Sural nerve
.0078 3.31 ⫻ 10⫺6 .8093* .0226 .0003
Zone 2
Zone 3
.0001 3.79 ⫻ 10⫺8 .0294 .0002 .0201
.0002 2.26 ⫻ 10⫺11 .4250* .0018 5.08 ⫻ 10⫺5
Zone 4 1.32 ⫻ 10⫺11 1.74 ⫻ 10⫺15 3.78 ⫻ 10⫺11 8.59 ⫻ 10⫺12 .0103
*No statistically significant difference in number of fascicles between donor and recipient nerves.
wrist and proximal to the bifurcation, sural nerve grafts were the closest match but still had significantly fewer fascicles than the common digital nerve samples (p ⬍ .05).
Discussion Primary tensionless repair is the desired treatment for traumatically transected digital nerves. In the setting of segmental nerve loss resulting from avulsion, crushed and debrided margins, or contraction and scarring in delayed repair, interposition conduits are required to obtain a tension-free coaptation. The pursuit of alternative nerve conduits has been the subject of research exploring autogenous vascular4 and muscle5 and nonautogenous biologic6 and synthetic conduits.7 Despite these experimental data, autogenous nerve grafts remain the most common conduits used for segmental defects. Much of the literature on the use of autogenous nerve grafting addresses surgical technique and functional outcomes. Standard protocols for the selection of nerve donor sites for specific anatomic defects have not been developed. The surgeon must consider many variables when selecting donor nerves, including ease of harvest, visibility of donor site scar, magnitude of donor site sensory deficit, length of available nerve, and caliber of the donor nerve. Harvestable length of the various donor nerves is well described in peripheral nerve surgery literature.8,9 Each available donor site offers the surgeon specific qualitative advantages and disadvantages. Anterior interosseous nerve and PIN, although more difficult to harvest, result in negligible donor sensory deficits. The dorsal forearm scar is minimal but conspicuous. Medial antebrachial cutaneous nerve and LABCN are harvested easily and leave less conspicuous scars, but the donor sensory deficit is more notable. Use of the anterior branch of MABCN avoids sensory losses to the elbow and posterior proximal fore-
arm, the crucial dermatome supplied by the posterior branch of the same nerve.10 Harvest of LABCN leaves a sensory deficit in the radial aspect of the volar forearm, but the dermatome overlap with the sensory branch of the radial nerve in 75% of the population minimizes this deficit.11 The sural nerve provides the greatest available length of 30 to 40 cm. The donor site scar can be unsightly, particularly if a longitudinal approach is used rather than the stepladder transverse open approach or an endoscopic harvest. The harvest is not technically difficult, particularly when only a short length is required for nerve grafting in the hand. Loss of the sural nerve results in a sensory deficit on the lateral aspect of the foot that diminishes over time.5 In contrast to these qualitative considerations to selecting nerve graft harvest sites, guidelines for caliber matching of donor nerves to specific defects have not been established, perhaps because no studies have examined the impact of nerve graft caliber matching on functional outcome. The importance of selecting nerve grafts of similar caliber to the defects remains intuitive. A caliber-matched nerve interface leads to more accurate technical repair. Subsequently the interface requires minimal sutures and perhaps results in less scarring to achieve alignment. Whether fewer sutures and less scarring achieve superior sensory recovery requires future investigation. Cable grafting is a method of achieving reasonable caliber matching between segmental nerve defects and interpositional grafts. The disadvantage of cable grafting often cited is the large percentage of the graft surface area that is not in contact with the recipient bed. Revascularization of these free grafts is through spontaneous anastomoses between vessels in the recipient bed and the vessels on the graft’s surface. It is believed that the central portions of cable grafts are unlikely to be revascularized and the endoneurial pattern of the Schwann cells is more
The Journal of Hand Surgery / Vol. 27A No. 2 March 2002 291
Table 3. Appropriate Donor Nerve Graft Sites by Anatomic Zone: Cross-sectional Area and Fascicle Number Zone 1
Zone 2
Zone 3
Zone 4
Cross-sectional area
AIN* PIN* MABCN*
LABCN*
LABCN*
Sural nerve
Number of fascicles
LABCN*
LABCN
LABCN*
Sural nerve
*Bold lettering signifies statistical significance.
likely to become fibrotic.9 Cable grafting is technically more difficult, requiring more sutures for alignment and potentially resulting in greater scarring at the graft interface. The choice of cable grafting rather than using a graft of appropriate diameter requires greater length of donor nerve, potentially resulting in greater dissection and larger harvest scars. Although the technique of cable grafting may be used to avoid other comorbidities of specific donor sites (ie, sensory loss at the lateral aspect of the foot), we believe that conventional nerve grafting with well caliber matched donor nerves most likely results in favorable outcome and we reserve cable grafting for occasions when appropriate size single grafts are not available. We used an anatomic zoning system to detect precise changes in caliber and number of fascicles along the pathway of the digital nerve from the wrist to fingertip (Table 3). Samples within this zoning system were matched accurately with donor nerves. In this manner guidelines were developed for donor nerve selection for each zone according to the same 2 criteria of caliber and number of fascicles. When grafting defects in zone 4 (the common digital nerve proximal to the bifurcation), the sural nerve is the most appropriate choice of donor nerve. Although displaying considerably fewer fascicles and smaller cross-sectional area than the zone 4 digital nerve, it is the closest available match among the selected donor nerves in both criteria examined. Lateral antebrachial cutaneous nerve is the best match for deficits in zones 2 and 3 (from the fingertip trifurcation to the common digital nerve bifurcation). There is no major difference in the caliber match between LABCN and the digital nerve in zones 2 and 3. The fascicle numbers also are equivalent for zone 3, and although LABCN has markedly fewer fascicles than zone 2 nerves, it is the closest match among the donors studied. Lateral antebrachial cutaneous nerve can be har-
vested with ease, and its resultant sensory deficit overlaps with the dermatome of the sensory branch of the radial nerve.8 In the fingertip distal to the digital nerve trifurcation (zone 1), AIN, PIN, and MABCN all are appropriate choices for caliber-matched grafts. Although these donor nerves display caliber similarity to the zone 1 defects, LACBN is the only similar donor nerve when number of fascicles is assessed. This is the only zone where the 2 comparison criteria produced different results. The relative importance of these 2 nerve graft criteria (cross-sectional area and number of fascicles) to sensory outcomes has not been studied. This is an area of future investigation to provide guidance in selection of donor nerve grafts in this region. In our institution zone 1 defects are repaired by caliber matching as the criteria for donor nerve selection for 2 reasons: (1) We perform standard epineurial repair without consideration for anatomic alignment of fascicles in that region, and (2) grafting in this zone is technically demanding and can be simplified by use of a well caliber matched graft. Among the caliber-matched options, we commonly select PIN as a zone 1 conduit for its ease of harvest and minimal donor site sensory deficit.
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