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Digital nerve repair- Reconstruction with Axogen Avance processed nerve allograft

    Overview

    Learn the Digital nerve repair: Reconstruction with Axogen Avance processed nerve allograft surgical technique with step by step instructions on OrthOracle. Our e-learning platform contains high resolution images and a certified CME of the Digital nerve repair: Reconstruction with Axogen Avance processed nerve allograft surgical procedure.
    This case demonstrates the technique of bridging a small nerve gap with a processed nerve allograft.
    Digital nerve injuries are a common consequence of penetrating hand injuries and are typically managed by direct microsurgical repair following exploration and wound debridement. The absence of tension across a direct epineurial nerve repair is an important factor in the success of digital nerve repair.
    In many cases this is difficult to achieve due to segmental loss, scarring and retraction in missed injuries, or following the required debridement of crushed or contaminated nerve ends.
    In such scenarios autologous nerve graft may be used to bridge a defect and provide an environment for nerve regeneration across a gap. The disadvantages of autograft include increased operative time and cost as well as donor site morbidity which include painful neuroma formation and sensory loss. This technique is covered elsewhere on the OrthOracle platform Nerve grafting for digital nerve defect (autograft using medial cutaneous nerve of forearm)
    The search for alternative strategies to avoid autograft is ongoing. Several studies have reported upon the success of artificial nerve conduits for bridging short gaps and one such technique is covered elsewhere on the OrthOracle platform Digital nerve conduit-assisted repair with the NeurolacTM (Polyganics)
    The use of nerve allograft to bridge gaps has demonstrated reasonable efficacy. Unprocessed nerve allografts will contain cellular material and immunosuppression is required until the nerve has regenerated and antigenic and cellular material has been replaced by the host. This is a major drawback and poses a significant risk of infection and other side effects from immunosuppressant therapy in otherwise healthy individuals.
    Development of processed nerve allograft has removed the need for immunosuppression. AxoGen Inc is a biomedical device company that has proprietary technology for the removal of cellular debris and neuroinhibitory glycosaminoglycans (GAGs) including chondroitin from the donor nerve material. A series of enzyme washes and subequent sterilisation with gamma irradiation preserve the mechanical integrity of the endoneural tube structure of peripheral nerves. Sections of varying diameters (1-2mm/2-3mm/3-4mm/4-5mm) and lengths (15mm/30mm/50mm/70mm) are provided frozen and can be rapidly defrosted in warm saline prior to use.
    The AVANCE TM processed nerve allograft (Axogen Inc. Alachua, Florida, USA) has demonstrable efficacy in short sensory nerve gaps that is comparable to autologous nerve grafts. the evidence for longer gaps and for mixed motor-sensory nerve reconstruction is more limited. A post-marketing surveillance registry study (RANGER) continues to collect results of allograft use internationally ad publishes regular reports on efficacy in the medical literature.
    In the UK, the National Institute for health and care Excellence (NICE) have outlined (in IPG597) that there is enough evidence to support the use of processed nerve allograft in digital nerve gaps provided that standard arrangements are in place for clinical governance, consent and audit.
    However, due to limited evidence for indications other than digital nerve repair, NICE recommend that for these cases, processed nerve allograft only be used with special arrangements for clinical governance, consent and audit or research.


    Indications

    INDICATIONS
    A digital nerve injury must be suspected whenever a sharp laceration presents with a loss of distal sensation. The wound is then explored under magnification and the digital nerve visualised. In an acute injury, the nerve ends may be directly coapted with a tension free epineurial repair technique.
    The absence of tension across a direct coaptation is an important factor in the success of digital nerve repair.
    In many cases this is difficult to achieve due to segmental loss, scarring and retraction in missed injuries, or following debridement of crushed or contaminated nerve ends.
    In this late presenting nerve injury, a nerve gap was bridged with processed nerve allograft.
    SYMPTOMS & EXAMINATION
    The commonest presentation of a digital nerve injury is following a sharp laceration to the hand. This will be accompanied by numbness at the finger tip but this may not be immediately apparent in the context of a painful laceration and preserved sensory input from other intact territories. The presence of a nerve injury should therefore be sought out by careful examination.
    The absence of light touch sensation by stroking the relevant side of the pulp.
    Two point discrimination is often be used. A normal sensation is reported as 1-5mm. In recent years two point discrimination has been criticised for confusing temporal and spatial stimulation and for its low sensitivty in nerve injury.
    We often use the ‘ten test’ where the patient is asked to score the sensation at the pulp in comparison to an uninjured area that scores a baseline of 10/10.
    In this patient where the diagnosis was made late after the wound had healed there was a clear Tinel’s sign indicating the location of the proximal nerve end.
    IMAGING
    Imaging is not commonly used in the diagnosis of acute digital nerve injury but in a longstanding injury where a neuroma is suspected and ultrasound scan or MRI scan may help to delineate the anatomy.
    ALTERNATIVE TREATMENT
    Leaving the nerve unrepaired may result in some recovery of protective sensation over 6 months but may also cause a painful neuroma at the site of injury. This may be considered in patients unfit for surgery, or unwilling to undergo wound exploration.
    Other methods to bridge a nerve gap include autograft. Auto graft s typically harvested from a more expendable donor sensory nerve in the same limb. Long lengths of autologous nerve may require lateral cutaneous nerve of forearm (100-120mm), medial cutaneous nerve of forearm harvested in the upper arm (120-160mm) or sural nerve (280-350mm). Small segments (20-30mm) can use the terminal branches of the posterior interosseus nerve on the dorsal aspect of the wrist (4th extensor compartment), however there is low endometrial tube density and the donor nerve is of poor quality for critical sensory gap reconstruction in the hand. Nerve conduits may be considered for small gaps of 5-12mm although some report reasonable outcomes up to 25mm. there are numerous conduits available for short nerve gap reconstruction including collagen conduits, polycaprolactone and chitosan.
    CONTRAINDICATIONS
    Apart from the general contraindications to surgery, a heavily contaminated wound is not suitable for allograft until adequately debrided.

    Setup

    The patient underwent regional anaesthesia by means of an axillary nerve block.
    A standard operating arm table and upper arm tourniquet were used.
    Bipolar diathermy, micro-instruments, loupe magnification and an operating microscope are all essential.

    Operation

    Positioning the hand to ensure adequate access throughout the procedure is a key step
    Following the initial patient checks, the skin is prepped to the elbow. The surgical drapes are applied followed by exsanguination and inflation of the tourniquet to 250mmHg.
    The hand is placed supinated on the arm table with the lead hand serving to hold the fingers in extension.
    The pressure exerted by the lead hand should be gentle and some padding used where necessary.

    Skin is marked using standard Brunner-type incisions across the joint linesThe incisions are planned.
    The clinical examination suggests that the ulnar digital nerve will need exposure proximal and distal to the healing wound.
    A series of half Brunners incisions are marked out, incorporating the ulnar portion of the original wound as demonstrated here.
    These incisions with the point of each triangle placed at the joint crease, allow one to access structures across joint creases without risking a joint contracture.
    Unlike a full Brunners incision these do not cross the entire width of the digit preserving some of the volar venous drainage.

    Skin incisions madeA skin knife is used to incise through the dermis and to then raise each flap.
    Bipolar diathermy is used to cauterise vessels as they come into view.

    Full thickness skin flaps are raisedSkin hooks are used here to provide retraction.
    Full thickness flaps are raised to avoid de-vascularising the skin edges.

    Stay sutures allow access with minimal retraction from the assistantThe flaps may now be sutured down to the skin on either side aiding exposure and reducing the need for retractors in the operative field.

    Careful exposure of digital neurovascular structuresBoth neurovascular bundles are exposed by blunt dissection using tenotomy scissors, They are found within the fat layer, ulnar and radial to the flexor tendon sheath. They are situated in the volar aspect of this fat and are prone to iatrogenic injury during exporation.
    After ensuring that the radial neurovascular bundle is intact, the ulnar neurovascular structures are exposed over a longer length.
    The digital nerve is the most volar structure with the artery and vein situated more dorsally

    Exposure of the digital nerve endsInitial blunt dissection with tenotomy scissors reveals discontinuity of the ulnar digital nerve, artery and vein.
    These are further dissected out using jewellers forceps and curved microscissors.
    The lacerated edge of the distal stump is clear but the proximal stump is less well defined, following attempted regeneration across a gap filled with fibrous scar tissue.
    A – Proximal nerve end
    B – Distal nerve end
    C – Flexor sheath

    Debridement of proximal digital nerve stumpA blade is used to trim the proximal end to look for any fascicle structure.
    A number of thin slices are taken sequentially through the scar tissue until normal fascicle structure is encountered. This serves to to minimise the final gap whilst ensuring the proximal end offers viable potential for regeneration.

    Neurotomy prior to repairOnce some fascicle structure with the presence of endoneurial vasculature has been identified, a final fine cut is taken with a neurotome. This ensures a clean cut with minimal distortion of the fine structure of the nerve end.
    The distal stump must also be examined and any scarred end trimmed away.
    A – 2mm Neurotome
    B – Neurotome blade
    C – Proximal nerve

    Measurement of the gap prior to allograft selection.With the digit fully extended the final gap is measured as 1cm.
    Background material is placed behind the nerve ends to improve visualisation.

    Irrigation of surgical fieldThe wound is irrigated with normal saline.

    Allograft is placed into the gap to gauge the length required. An appropriate diameter of processed nerve allograft is requested from the tissue bank.
    This is a 2-3mm Avance allograft and the excess length will be trimmed to bridge the gap without tension.
    The putative advantage of processed nerve allograft over conduit devices is its preserved extracellular matrix which should act as a scaffold to support axonal growth across the gap. It is therefore important to ensure the allograft is not handled directly to protect its structural integrity.
    Here the forceps are used to hold the allograft at a site that will not form part of the final bridge.

    Microsope set up and accessThe microscope is now brought into the field.
    Microscope focal length, inter-pupillary distance and access to the arm table, should all be set up prior to starting the operation. This ensures that no tourniquet time is wasted at this stage configuring the microscope.

    Ensure correct orientation of the nerve graft Despite the nerve gap, the proximal and distal stumps are placed in the correct orientation to each other, ensuring there is no twist at either end.
    The proximal end of the allograft is now sutured to the proximal stump.
    A 9/0 nylon suture is used to perform an epineurial repair.
    Nerve and graft handling can be kept to a minimum by using the suture needle and suture material to aid the final approximation of the two ends.
    A tight approximation risks distorting fascicle alignment and leading to overlap at the coaptation. It is preferable to leave the nerve ends just touching, with sutures more loosely applied than they would be for example in the context of a microvascular anastomosis.

    Epineural repair using 9/0 nylon suture.The back wall of the proximal end may be accessed as shown here.
    Since there is no tension in this scenario, three or four sutures are usually adequate.

    Cutting allograft to its final length.The distal end of the graft is now trimmed to length using a sharp blade or a neurotome.
    Enough allograft length must be preserved to ensure there is no tension across the construct with the digit fully extended.

    Distal epineural suture placementThree or four epineurial sutures are placed distally.
    The dorsal sutures in this case are placed as the assistant gently rotates the repair to one side.

    Inspection of construct and assessment of its tension.The final construct is shown here.
    The digit is in full extension demonstrating a tension free reconstruction.

    Final irrigationThe background material is removed and the wound is now irrigated.

    Closure of wound-removal of stay suturesThe stay sutures are cut and the flaps placed back over the nerve reconstruction.

    Closure of wound-Skin closure5/0 nylon is used to approximate the wound edges with interrupted skin sutures. The corner stitches are usually placed first.
    Care is taken to avoid damaging the allograft at this stage.

    Dressing applicationAfter cleaning the skin with normal saline, a non adherent dressing such as Jelonet or Mepitel is applied.

    Dressing applicationBlue gauze strips are placed over this, and within the web space.

    Dressing applicationA velband and crepe wrapping is applied.
    Plaster immobilisation is not used.

    Post Operative

    The patient is dicharged home the same day with adequate oral analgesia and a Bradford sling to keep the limb elevated.
    The digit is not splinted.
    The wound is checked in an outpatient dressing clinic at 1 week and a lighter dressing is applied. Gentle finger movements are commenced at this stage to prevent stiffness.
    At 10-14 days weeks the sutures are removed and the patient is allowed to return to light activities. Normal activities such as driving may be resumed between 2 and 4 weeks as pain allows.
    Nerve regeneration typically takes place over a period of time that equates to 1mm of regeneration per day. In our experience regeneration through an allograft may take longer than this.
    A progressive Tinel’s sign is highly suggestive of ongoing regeneration and the patient should be monitored at 6-8 weekly intervals to ensure recovery continues according to the expected timeframe. Distal sensation should be checked at each follow up appointment. Worsening neuropathic pain associated with a non-progressing Tinels suggests arrested regeneration which may merit further treatment including re-exploration.
    A consistent outcome assessment of results is important. Static two point discrimination is the most commonly reported outcome measure in the literature and therefore allows comparison with other studies. As mentioned above however it is increasingly regarded as a unreliable measure. Monofilament testing may be more reproducible. A patient reported outcome measure is perhaps a more relevant measure of the success of surgery.

    Recommended Reading

    CHOW, S. P., & NG, C. (1993). Can a Divided Digital Nerve on One Side of the Finger be Left Unrepaired? Journal of Hand Surgery, 18(5), 629–630. doi:10.1016/0266-7681(93)90020-g
    A prospective comparison of repair vs non repair of digital nerve injuries showed significantly better results in the repaired group. Improvement was demonstrated over two years post repair, with 90% achieving two point discrimination compared to only 6% in the non-repaired group.
    Patients with poorer long term outcomes due to an un-repaired nerve were not subsequently willing to undergo further surgery to attempt to treat a painful digital neuroma or poor sensation.
    Rinker et al. Use of Processed Nerve Allografts to Repair Nerve Injuries Greater Than 25 mm in the Hand.Ann Plast Surg. 2017 Jun;78(6S Suppl 5):S292-S295
    The RANGER database is an industry registry of outcomes for processed nerve allograft. A subset analysis for digital nerve injury with gaps of 25mm or greater demonstrated recovery to S3 level in 86% of repairs which compares favourably to historical data using autograft (60-88%). The authors point out the lack of donor site morbidity or adverse reactions to the allograft.
    Clare TD et al. Rehabilitation of digital nerve repair: is splinting necessary? Hand Surg Br. 2004 Dec;29(6):552-6.
    This study of splinting of digital nerve repairs demonstrated no advantage to eventual sensibility. Non-splinted patients returned to work earlier and splinting was associated with increased stiffness in the early period.
    Dickson et al. Nerve allograft reconstruction of digital neuromata. J Musculoskelet Surg Res 2019;3:116-22
    Our own series of processed nerve allografts used to treat 12 digital neuromas in 10 patients, demonstrated satisfactory outcomes in 80%.
    Taras, J. et al. Allograft Reconstruction for Digital Nerve Loss. The Journal of Hand Surgery, 38(10), 2013.
    1965–1971. doi:10.1016/j.jhsa.2013.07.008
    A study of allograft used to reconstruct digital nerve defects of up to 30mm demonstrating safety and clinical efficacy.

    Reference

    • orthoracle.com

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