If you don't remember your password, you can reset it by entering your email address and clicking the Reset Password button. You will then receive an email that contains a secure link for resetting your password
If the address matches a valid account an email will be sent to __email__ with instructions for resetting your password
Fifteen fresh human cadaver hands were dissected, using ×2.8 loupe magnification, to study the subcutaneous innervation at the site of the incision (in the line with the radial border of the ring finger) for standard open carpal tunnel decompression. Subcutaneous nerve branches were detected and traced proximally to determine their origin. Morphometric analysis of nerve cross sections from the site of the incision and from the main nerve trunk proximal to cutaneous arborisation was performed using light and transmission electron microscopy and a computer-based image analysis system. At the site of the incision, the ulnar sub-branch (US) of the palmar cutaneous branch of the median nerve (PCBMN), which innervates the skin over the hypothenar eminence, was found in 10 of 15 cases. Branches from the ulnar side were not detected. The main trunk of PCBMN consisted on average of 1000 (SD 229) myelinated axons arranged in 1–4 fascicles. In the US of the PCBMN there were on average 620 (SD 220) myelinated axons, 80% of them smaller than 40 μm2 i.e. thin myelinated axons, and on average 2037 (SD 1106) unmyelinated axons, arranged in 1–3 fascicles. The ratio of the number of myelinated axons in the US and the main trunk of the PCBMN was on average 63% (SD 19%). Frequency distribution of cross-sectional areas of myelinated axons shows no significant difference between the US and the main nerve trunk of the PCBMN. The importance of incision trauma to subcutaneous innervation of palmar triangle is emphasised and possible mechanisms of scar discomfort are discussed.
The cutaneous innervation of the palmar triangle is of interest since it is the most frequently incised place in hand surgery. Incisions in the proximal palm are used to approach the carpal tunnel, distal radius, scaphoid and wrist ganglions. Open release of the transverse carpal ligament is a standard for the treatment of carpal tunnel syndrome.
However, incision trauma in this area is commonly associated with post-operative scar discomfort that delays patient's return to routine activities. Inadverent division of the palmar cutaneous branch of median nerve (PCBMN) and/or palmar cutaneous branch of the ulnar nerve (PCBUN) or their larger branches with subsequent neuroma formation is a possible cause of painful scarring.
To reduce postoperative morbidity, after carpal tunnel surgery, alternative techniques (endoscopic, twin incision, limited vision) leaving the skin over the heel of the hand intact have been proposed.
Even though patients may return to work more quickly after these procedures, proponents of the standard open approach were reluctant to accept the higher rate of complications that occurred using these blind methods.
A recent prospective clinical study on 416 hands revealed, that preservation of the subcutaneous innervation during the open release of the transverse carpal ligament significantly lessens the incidence of post-operative scar discomfort. In the same study ulnar sub-branches (US) of the PCBMN were detected at the site of the incision for carpal tunnel decompression in 146 of 200 hands and were classified in four anatomical variations.
The aim of the present study on cadaver hands was to asses the frequency and origin of nerve branches that are frequently transected in carpal tunnel surgery. In addition, their composition regarding the number and cross sectional area of myelinated axons was determined using light microscopy, and the number of unmyelinated axons was counted using electron microscopy.
1. Materials and methods
The study was approved by the National Committee for Medical Ethics. Fifteen fresh human cadaver hands (seven male and eight female), without identified deformity were dissected using ×2.8 loupe magnification. Mean age was 53 years (range, 27–80). Three pairs of hands were from the same donors and the remaining nine hands from different donors. The skin was cut with 3 cm long incision in the line with the radial border of the ring finger originating from the distal wrist crease.
Subcutaneous nerve branches were separated from the subcutaneous fat and underlying palmar fascia, and traced proximally to the main nerve trunk (PCBMN/PCBUN). Thereafter, the incision was elongated proximally to the forearm to identify the origin of the main nerve trunk from the ulnar or median nerve (Fig. 1) .
Fig. 1(A) Palmar triangle and marking for the incision in the line with the radial border of the ring finger originating from the distal wrist crease. (B) Dissection of the palmar cutaneous branch of the median nerve demonstrating its origin from the median nerve (∗) and its cutaneous arborisation in the palmar triangle. (C) Main trunk of the palmar cutaneous branch of the median nerve (MT) and ulnar sub-branch (SB) with its terminal branches that are frequently endangered during surgery in the proximal palm.
After identification, 2 mm long segments of the PCBMN and of its thickest US at the site of the primary incision were excised. Excised nerve segments were fixed for 24 h in Mc Dowell fixative buffered with Milloning's solution to pH 7.2–7.4, immersed for at least 6 h in Milloning's solution. Post fixation was done in 2% OsO4 containing veronal acetate buffer and sucrose. The blocks of tissue were dehydrated in ascending grades of ethanol, cleared in propylene oxide and embedded in Epon 812. Semi thin sections (0.5–2 μm) were cut with LCB Ultracut III stained with Azur II, and analysed by light microscopy (Fig. 2) . Ultra thin sections (30–50 nm) were cut, mounted on coated whole grids, stained with saturated uranyl-acetate and with lead-citrate and examined by Jeol 1200 EX II transmission electron microscope.
Fig. 2Light photomicrographs of the cross-sections of the ulnar sub-branch of the palmar cutaneous branch of the median nerve from the site of the incision in the line with the radial border of the ring finger. (A) Three fascicles are shown at ×30 magnification. (B) Myelinated fibres are presented at ×600 magnification.
Morphometric analysis of nerve cross sections was performed by a computer-based image analysis system with the Microcomputer Imaging Device program, which allowed us to digitalise the picture obtained by a light microscope (Zeiss–Opton). The myelinated axons were circumscribed on the computer screen at ×1000 magnification; then the axonal counts and their cross-section areas were calculated by the morphometry software. The number of unmyelinated axons were counted on electron micrographs of five specimens, taken at ×1000 and projected at final magnification of ×5000 (Fig. 3) . In accordance with most authors, unmyelinated axons were considered to be all profiles meeting two or more of the following criteria: rounded profiles surrounded by Schwann cell processes and exhibiting a distinct mesaxon, lighter appearance of axoplasm compared to Schwann cell cytoplasm, larger number of microtubules in axoplasm than cytoplasm, and greater electron-density of the axolema compared to the Schwann cell plasmalema.
In addition, the ratios of number of unmyelinated towards myelinated axons of the two nerve branches were calculated.
Fig. 3Electron photomicrogaphs of the cross-sections of the ulnar sub-branch of the palmar cutaneous branch of the median nerve from the site of the incision in the line with the radial border of the ring finger at ×500 (A) and ×5000 (B) magnification.
At the site of the incision in the line with the radial border of the ring finger originating from the distal wrist crease, subcutaneous nerve fibres were detected in 10 of 15 specimens. All identified nerves originated from the radial border of the incision and were subbranches of the PCBMN. In 2 of 10 cases more than one US was identified.
In the main nerve trunk of the PCBMN there were on average 1000 (SD 229) myelinated axons arranged in 1–4 fascicles. US of the PCBMN consisted on average of 620 (SD 220) myelinated axons i.e. mean 63% (SD 19) of the number of myelinated axons in the main nerve trunk (Table 1) , and 2037 (SD 1106) unmyelinated axons, arranged in 1–3 fascicles. The ratio of the number of the unmyelinated towards myelinated axons in the US of the PCBMN was on average 3.7 (SD 0.3) (Table 2) . Analysis of cross-sectional areas of myelinated axons in the PCBMN and its US showed that cross-sectional areas of on average 806 (SD 162) and 513 (SD162) axons, respectively, were smaller than 40 μm2 (diameter <7 μm). Frequency distribution of cross-sectional areas of myelinated axons showed no significant difference between the US and the main nerve trunk of PCBMN (Fig. 4) .
Table 1The number of fascicles (f) and myelinated axons (MA) in the ulnar sub-branch (US of PCBMN) and the main trunk (PCBMN) of the palmar cutaneous branch of the median nerve. The ratio of number of myelinated axons in ulnar sub-branch and main nerve trunk is presented
Table 2The number and ratio of unmyelinated (UA) and myelinated (MA) axons in five samples of the ulnar sub-branch of the palmar cutaneous branch of the median nerve from the site of the incision in the line with the radial border of the ring finger
Fig. 4Frequency distribution histogram of cross-sectional areas of myelinated axons in the palmar cutaneous branch of the median nerve (PCBMN) and its ulnar sub-branch (US). Samples of PCBMN were taken proximal to its cutaneous arborisation and samples of US were obtained from the site of the incision in the line with the radial border of the ring finger. There is less myelinated axons in the US, but distribution of cross-sectional areas is comparable.
The PCBMN is a constant structure. At the level of the distal wrist crease the PCBMN enters a tunnel located 3 mm ulnar to the thenar crease and than arborises in different directions. With the thenar incision there is 100% chance of injuring the PCBMN or its subbranch.
A medial (ulnar) sub-branch of the PCBMN, destined for the skin of the hypothenar region, has been observed in 42% of hands, never reaching beyond a line corresponding to the ulnar border of the 4th metacarpal.
At the most commonly studied landmark, the axial line of the ring finger, subbranches of PCBMN have been detected in 8–12% of cases using ×3.5 loupe magnification. These studies also proposed that injury to the PCBUN may occur as frequently or more frequently than injuries to the PCBMN when using the approach in the line with the axis of the ring finger.
However, thenar crease anatomy and ring finger projections are highly variable both in absolute location and configuration, providing a poor basis for incision placement.
In the present study on cadaver specimen, subcutaneous branches were identified, using ×2.8 loupe magnification, at the site of the incision, in the line with the radial border of the ring finger, in 10 of 15 cases, which is comparable with results of a clinical study where subcutaneous innervation has been detected in 73% of cases using the same incision and magnification.
All of the identified nerves crossed the path of the incision in radial to ulnar direction and were US of the PCBMN. These observations suggest that palmar cutaneous branches that are frequently endangered by the incision in line with the radial border of the ring finger derive from the median nerve.
We have found no report in the literature regarding nerve fiber composition of PCBMN. The results of our study suggest that the main nerve trunk of PCBMN proximal to cutaneous arborisation consists of 1–4 fascicles or approximately 1000 myelinated axons. The quantitative comparison of axons in the main trunk of PCBMN and its US revealed that about 63% of nerve fibers of the PCBMN turn ulnarward and form the US of PCBMN, which consists of 1–3 fascicles with approximately 619 myelinated axons. Since nerve samples were taken only from the thickest branch and in two of 10 cases we identified multiple branches, the actual number of endangered nerve fibers is even higher. These results confirm that US of the PCBMN cross the path of the incision for median nerve decompression and probably innervate the skin over hypothenar eminence.
More than 80% of cross-section areas of myelinated axons in the PCBMN and its US were found to be smaller than 40 μm2. According to Boyd and Davey, these myelinated fibers belong to the subclass of the smallest myelinated axons (A-δ), which are associated with unencapsulated free nerve endings that transfer the sensation of sharp, pricking pain, tickling and temperature.
Electron microscopic analysis revealed that US of the PCBMN consists of approximately 2037 unmyelinated axons (C-fibers), which are somatic afferent and autonomic (sudo- and vasomotor) fibers. Unmyelinated axons outnumber the myelinated approximately 3 to 4-fold, which is consistent with the data of Berthold and Ryamark.
These findings imply that during surgery in the palmar triangle about 2700 axons may be transected. The axon sprouts at the proximal end of the transected nerve multiply by at least 10-fold.
This can lead up to 30,000 nerve sprouts, entrapped within the fibrous tissue of the scar, at the mechanically exposed area of the proximal palm. The reported incidence of scar discomfort after open carpal tunnel decompression ranges from 11 to 61%
and can be partially explained by the number of injured nerve fibers. Subcutaneous nerve branches may also be injured by coagulation, or suturing during open techniques and even by channel preparations for endoscopic procedures.
In addition to nerve trauma there is a considerable number of other possibilities for the development of pathologic pain after nerve injury. Incisional trauma to sensory axons often has the paradoxical effect of inducing positive sensory disturbances; paraesthesias and chronic neuropathic pain. Regenerating axon sprouts that may grow towards its target tissue, or are entrapped in the scar tissue or neuroma, form axon end bulbs that are capable of abnormal ectopic discharge. The formation of these ectopic neural pacemaker sites depends on axon type and location at which particular axon is injured. In addition to firing spontaneously, the ectopic sources in injured nerve become sensitive to mechanical, thermal, chemical stimulation and metabolic events. Locations where nerves run adjacent to tendon and bone, or where small branches cross over tough fascial planes are particularly at risk for developing consistently located mechanosensitive tender spots.
It has become clear in recent years that pathophysiological changes in the peripheral nervous system can induce biochemical and structural changes in the central nervous system leading to hypersensitivity and allodinia.
Whatever the pathophysiological mechanism, the number of transected axons, is a significant factor in pain generation.
During surgery in the proximal palm high number of nerve fibers can be injured. Unsuccessful regeneration of these fibers may explain painful scarring. We have found no description of US of PCBMN, which has a clinical relevance, in the classical anatomical textbooks. We emphasise the importance of incision trauma to subcutaneous nerves in the palmar triangle and recommend careful dissection with identification and preservation of subcutaneous nerve branches during operations in this particular area.
00418-1&id=gr1.jpg){kind=link}
00418-1&id=gr2.jpg){kind=link}
00418-1&id=gr3.jpg){kind=link}
00418-1&id=gr4.jpg){kind=link}