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function of the gastrocnemius and soleus muscles following determined by its effect on the injured nerve stump and
denervation. The sensory‑protected group underwent secondarily by its effect on the muscle. Other studies
[29]
saphenous‑to‑tibial nerve transfer using end‑to‑end repair. have not largely substantiated these claims.
Compared to the unprotected group, gross and histological End‑to‑side neurorrhaphy
examination showed the sensory‑protected group had End‑to‑side neurorrhaphy provides trophic support from
higher muscle weight and greater preservation of muscle the donor nerve and enables the regenerated motor axons
structure, including less fiber atrophy and connective to reach their target, thus bypassing the need for a second
tissue hyperplasia. More importantly, the sensory‑protected operation to replace the motor nerve supply. In contrast,
rats demonstrated larger maximum compound action end‑to‑end neurorrhaphy requires cutting the donor
potentials and relative preservation of isometric force sensory nerve and suturing it to its native stump once
[9]
overtime. Follow‑up studies showed sensory protection the motor nerve has regenerated, essentially denervating
[23]
also prevented muscle spindle deterioration. Nonetheless, the muscle twice. Studies have shown that two cycles
rats that underwent immediate nerve repair had the best of denervation and reinnervation lead to suboptimal
structural and functional outcomes.
functional recovery, as demonstrated by reduced muscle
The benefits of sensory protection have been power and force. [31]
substantiated by other groups. [24‑26] Common outcomes End‑to‑side coaptation is a compelling alternative
of end‑to‑end sensory nerve grafting in the lower limbs when conventional end‑to‑end coaptation is not
include preservation of fiber distribution, maintenance practical. Such cases occur when the transected end
of motor endplates, and less muscle atrophy, fibrosis, of the motor nerve stump is far from its muscle
collagenization, and fat deposition. [16,25] Consistent with target or when there are multiple denervated muscles.
lower extremity studies, Beck‑Broichsitter et al. found Zuijdendorp et al. recently investigated the efficacy
[11]
[32]
that sensory‑protection in the upper extremity resulted of sensory protection using end‑to‑side neurorrhaphy.
in higher muscle weight, larger axon diameter, and The investigators sutured the divided end of the sural
larger nerve fiber surface area. However, there was no nerve to the lateral aspect of the tibial nerve stump
definitive difference in grasping strength between the and examined the gastrocnemius muscle at 5 weeks
sensory‑protected and unprotected groups. [11]
and 10 weeks postoperatively. Compared to primary
The protective effect by a purely sensory nerve can be end‑to‑end neurorrhaphy, the sensory‑protected group
explained by a number of factors. First, the trophic effect demonstrated a statistically significant increase in muscle
of the sensory nerve provides a supportive milieu for the weight and decrease in muscle atrophy compared to the
maintenance of skeletal muscles. More specifically, sensory unprotected group. Despite the ongoing debate over
[32]
protection modulates the expression of both glial cell the efficacy, end‑to‑side neurorrhaphy remains a viable
line‑derived neurotrophic factor (GDNF) and brain‑derived approach that requires further investigation.
neurotrophic factor (BDNF), thereby optimizing the Researchers have also used the end‑to‑side neurorrhaphy
environment for muscle preservation and reinnervation. [12,27] model to study the protective effects of mixed nerve
Second, the sensory nerve helps maintain the architecture containing both motor and sensory axons. The use of
of the residual nerve stump and basal lamina of endoneurial mixed nerve is supported by a recent study by Li et al.
[33]
tubes, thus facilitating later access by regenerating motor who compared muscle protection following denervation
axons. [4,28] Third, Schwann cells of the distal stump switch using the peroneal nerve (mixed protection) or sural
from a myelinating phenotype to a growth‑supporting nerve (sensory protection). They showed that both the
phenotype by upregulating specific genes associated with mixed‑ and sensory‑protected groups demonstrated
regeneration. These trophic effects help maintain a preservation of muscle architecture and better functional
[19]
growth‑supportive environment, which would otherwise recovery following reinnervation compared to the
deteriorate with time.
unprotected group. Further, the investigators showed
Although the research overwhelmingly supports the that mixed protection was superior to sensory protection
benefits of sensory protection, it is important to in terms of axon structure (more regenerated myelinated
recognize points of contention. Sulaiman et al. axons, larger axonal diameter, thicker myelin sheath) and
[29]
proposed that the presence of sensory nerves instead function (greater contraction force). They controlled
[33]
create an unfavorable environment that reduces motor for stump reinnervation by the motor component of
axonal regeneration and myelination of regenerated the mixed nerve by performing end‑to‑side coaptation
axons. The authors advocated that sensory nerves actively and capping the end of the transected motor nerve. In
inhibit motor axonal regeneration by not only physically contrast, another study by Michalski et al. reported
[27]
occupying the endoneurial pathways, but by altering no difference between mixed‑ and sensory‑protected
Schwann cells in the distal nerve stump. Specifically, groups in terms of number of regenerating axons, axon
sensory nerves shift Schwann cells toward a phenotype diameter, and myelin cross‑sectional area in the distal
that is less receptive to regenerating motor axons, in stump. However, there was a difference in the expression
part by down‑regulating the expression of L2/HNK‑1 of denervation‑induced GDNF, and BDNF expression
required for interaction between Schwann cells and motor between the two groups, which suggests that the main
axons. [29,30] Furthermore, the investigators argue that the benefit of mixed protection is more rapid normalization
functional outcome of chronic denervation is primarily of trophic factors.
204 Plast Aesthet Res || Vol 2 || Issue 4 || Jul 15, 2015