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Supinator to Posterior Interosseous Nerve Transfer (S-PIN transfer)

    Overview

    Learn the Supinator to Posterior Interosseous Nerve Transfer (S-PIN transfer) surgical technique with step by step instructions on OrthOracle. Our e-learning platform contains high resolution images and a certified CME of the Supinator to Posterior Interosseous Nerve Transfer (S-PIN transfer) surgical procedure.

    Motor nerve transfer surgery uses an expendable nerve branch or redundant nerve fascicle from within a functioning nerve to transfer directly to the terminal motor branch of a paralysed muscle with a more important function. The nerve co-aptation is close to the target muscle and reinnervation distances are therefore short. The Supinator to Posterior Interosseus nerve transfer (S-PIN transfer) is indicated in isolated lower brachial plexus injuries (C8/T1), degenerate C8 motor radiculopathy or in mid-cervical tetraplegia. The aim is to restore digit extension using medial and lateral branches of the PIN to supinator innervated from the C6 root. These branches are implanted to the adjacent PIN (C8 digit extension) through either an anterior or a posterior approach to the PIN. The supinator is decompressed as part of the procedure when performed through the anterior approach and this technique is illustrated here.

    Indications

    INDICATIONS
    The supinator branches of the PIN arise separate from the main PIN and supply supinator just beyond its proximal edge. Two branches each 1.5-2cm long can usually be identified – one for each head. Supinator is mainly C6 root innervation. The PIN supplies digit extension with C8.
    The SPIN transfer is indicated as part of a C8,T1 avulsion for restoration of digit extension. It may also be used in a motor radiculopathy or C8 or in mid-cervical tetraplegia for restoration of finger and thumb extension. This transfer is only for root level pathology without injury to the posterior cord or radial nerve.
    Typically this would be used for tetraplegia patients at ICHT grade 2-4 to assist active digit extension. The biceps is also a supinator and should be working normally and not planned for transfer (sometimes biceps to triceps tendon transfer is preferred in a spastic flexed elbow to the typical deltoid to triceps tendon transfer or the theres minor to long head of triceps nerve transfer for restoration of elbow extension in triceps minus ICHT cases).
    This transfer must be completed in the first 9 months from injury if the extensors demonstrate lower motor neurone denervation on EDC and EPL electromyographic testing. If there is no denervation (UMN lesion) then the transfer can be completed years after the spinal cord injury as long as the hand joints are flexible.
    SYMPTOMS & EXAMINATION
    The patient will have an intact posterior cord and radial nerve but absent C8 contribution to this nerve. In such cases there is good contraction of the supinator with the elbow extended as well as functional supination with the elbow flexed through the biceps tendon insertion to the proximal radius. The strength of supinator should be assessed and the procedure should not be contemplated unless there is MRC grade 5 power of supination. The function of EDC, EDM, ECU, EIP, APL, EPB and EPL will typically be absent. In a C8 motor radiculopathy there may be some sparing of EIP or EPL and the transfer will need to target fascicles of the PIN other than those already functioning. In such instances there is a risk of temporarily losing some of this function and so proceed with caution.
    IMAGING
    Imaging is not necessary. MRI of the cervical spine and brachial plexus may confirm C8 and T1 root avulsions of degenerative compression of the C8 nerve root.
    Neurophysiology studies can confirm denervation of the potential target muscles with fibrillation potentials on electromyography (EMG)>
    In the tetraplegic upper limb the EMG is more challenging to interpret. Patients may have denervation changes due to anterior horn cell damage and subsequent Wallerian degeneration (ie a lower motor neurone lesion of C7 and C8 innervated muscles, however there is often sparing of the anterior horn cells below the level of the cord contusion in a narrow cervical spinal cord zone of injury. In such cases there may be no volitional motor activity but some of the axons arising from C8 may be preserved without complete Wallerian degeneration. In such cases the procedure could successfully target non-functioning but non-denervated (upper motor neurone lesion) muscle well beyond the typical 6-12 months period for successful reinnervation of a LMN lesion using nerve transfer. There may be sampling error on EMG and whilst pre-operative EMG is useful in planning it cannot replace the intra-operative stimulation necessary after surgical exploration of the recipient nerve.
    ALTERNATIVE OPERATIVE TREATMENT
    Tendon transfers may be considered for restoration of finger and thumb extension if there is a reliable donor muscle. Typically in a C8.T1 root avulsion there is no finger flexion and only weak wrist flexion and so tendon transfer options are limited. The brachialis muscle may be transferred for finger flexion and the FCR if sufficiently strong can be transferred to combined finger and thumb extension.
    In a tetraplegic patient, modification of the tenodesis response can be achieved by the House procedure and the Elk procedures in a midcervical tetraplegia. The brachioradialis can be transferred for finger flexion or key pinch through FPL and the wrist extensors act to create tenodesis of the digits. A tendon loop from extensor to extensor in adjacent digits passing deep to the extensor mechanism improves passive digit extension in wrist flexion and allows digit roll-up when the wrist extensor is activated. This rebalances the index particularly allowing the thumb to target the side of the PIPJ for key grip which is active, being powered through the brachioradialis to FPL tendon transfer. The FPL flexion postion is improved by partialFPLto extensor transfer and EPL reefing.
    NON-OPERATIVE MANAGEMENT
    There are no good options for digit extension other than using a glove splint with either elastic extension recoil strips or an out-rigger passive extension assist splint. These are cumbersome and not well-tolerated by patients. Both devices require active digit flexion and are therefore not useful in a C8,T1 root avulsion.
    CONTRAINDICATIONS
    Non-functioning supinator or weak biceps supination are contra-indications to the S-PIN transfer.

    Setup

    The patient can have a regional anaesthetic block at the supraclavicular level or a high axillary nerve block. No peripheral nerve top ups should be allowed in the vicinity of the radial nerve at the lateral arm. An alternative is to use general anaesthesia without neuromuscular blockade.
    The patient is positioned supine with the arm extended on an arm table. A tourniquet is essential because of the rich vascular networks around the elbow and in the proximal forearm. should be applied to the upper arm but not elevated until the limb is prepped and draped. This minimised the nerve ischaemic time allowing exposure of the PIN and the supinator branches without loss of conduction. Use an Esmarch bandage to exsanguinate the limb after limb preparation and draping. The hand should be positioned in supination using a lead hand.
    A nerve stimulator is required and a circuit with patient electrode. Ideally the stimulator should be a variable stimulation device with a frequency of 2Hertz. A stimulator needle is used with and arthroscopy camera drape over the needle connections so that a member of the non-scrubbed theatre team can control the stimulator and adjust stimulation as instructed by the lead surgeon.
    Micro surgical instruments are required including fine jewellers forceps and curved serrated nerve microscissors.
    Nerve suture is with 9’0 nylon on a microneedle and the co-aptation is reinforced with Tisseel (Baxter) fibrin glue.

    Operation – 1

    The arm is positioned on a side table with a tourniquet applied to the upper arm but not inflated.

    The limb is prepped and draped and then an Esmarch bandage used to achieve exsanguination. A lead hand can be used to position the wrist, forearm and hand to maintain supination and a stable operating site for the microsurgical component of the operation.

    The skin incision is marked from the lateral elbow extending distally towards the mid-point of the forearm.

    A curvilinear incision is made in the proximal forearm from the lateral elbow extending distally. Dissection in the fat identifies the cephalic vein.

    The skin edges are mobilised and the deep fascia opened at the volar edge of the brachioradialis, dorsal to the cephalic vein.

    Before continuing the deep dissection, the cephalic vein is identified and retracted medially. Lying in close proximity to the vein is the lateral cutaneous nerve of the forearm (LCNF) which must be protected. The LCNF should be mobilised and protected in a sloop.

    The fascia between the mobile wad (BR, ECRL and ECRB) and the forearm flexor-pronator mass is now developed.

    The brachioradialis is exposed so that the volar edge can be elevated with a Ragnall retractor. Note the yellow sloop around the LCNF

    A self-retaining Weitlaner retractor is placed in the wound and a Ragnall retractor used to gently elevate the BR. Under brachioradialis the fat should be carefully dissected distally to identify the superficial radial nerve. Do not develop the proximal extent of the wound yet because the nerve to ECRB is small and has a variable course in the fat arising from either the main radial nerve, the SRN or the PIN directly. Excess dissection or traction proximally can damage this nerve.
    The Jamieson scissors are demonstrating that there is a crossing fascia band and the temptation is to divide it, however first the ECRB branch (must be identified and protected).

    Note the ECRB branch in the fat with the SRN.

    Mobilise the distal BR and identify the SRN. Pass a fine tipped mixter forceps under the nerve in preparation for passing a sloop. The mixter is under the SRN. Next look for the ECRB branch. If it is not immediately identified in the fat adjacent to the SRN, trace the SRN proximally to identify the ECRB branch origin from either the radial nerve, the SRN or the PIN. The ECRB branch should lie in the same interval as the SRN.

    The blue sloop has been placed around the SRN. The mixter is under the ERCB branch which lies adjacent to the SRN under BR. The yellow sloop is identifying the LCNF in the superficial fat to ensure that no unnecessary traction or damage occurs during placement of self-retaining retractors.

    ECRB Br: ECRB Branch
    SRN: Superficial radial nerve
    LCNF: Lateral cutaneous nerve of forearm
    CV: Cephalic vein

    Nerve stimulator confirms contraction in the motor ECRB branch and that the sensory SRN has no contraction.

    Protect the ECRB branch and trace proximally to identify its origin, typically from the SRN. Tracing this further proximally identifies the take-off of the PIN. A vascular pedicle crosses the nerve in this interval. I use small Ethicon ligaclips to maintain haemostasis and rapidly expose the nerve branches without having to use bipolar diathermy. Expose the radial nerve proximally, pass a sloop and then follow distally to identify the origin of the PIN.

    Release fascial bands overlying the nerve to fully expose the SRN, the ECRB branch and the PIN.

    External neurolysis and careful division of crossing vessels allows exposure of the more deeply sited PIN.

    Use the scissors in the distal part of the exposure to develop the plane radial to the SRN. The supinator muscle is now seen. The PIN passes under the leading edge of supinator between the humeral and ulnar heads. Proximally great care must be taken because vascular pedicles, the PIN and the supinator branches of the PIN lie in this plane.
    S: Supinator

    The supinator and ECRB branch are proximally lifted together as they arise and a mixter is used to pass a sloop at this point. This provides a more robust point for gentle retraction with a sloop than the risk of traction on the ECRB branch alone which is much smaller in diameter.

    The more proximal blue sloop is around the SRN and the ECRB branch combined. Lifting this sloop allows the PIN take-off to be identified passing more deeply towards supinator. A deeper vascular pedicle obscures the PIN.

    VP: Vascular Pedicle

    The SRN and ECRB is retracted in an ulnar direction and the vascular pedicle is divided radially between Ethicon ligaclips. This image demonstrates application of the ligaclip.

    Carefully spread the scissors in this interval to identify the leading edge of the supinator muscle. Identify any further vascular pedicles and maintain meticulous haemostasis. The PIN and the supinator branches are in this fat passing deep to the supinator muscle.
    BR: Brachioradialis
    RN: Radial nerve
    ECRB: Extensor carpi radialis brevis
    ECRB Br: Nerve to ECRB
    SLE: Supinator leading edge
    PIN: Posterior interosseus nerve
    SBr: Supinator nerve branches x2

    Following division of the vascular pedicle between the two ligaclips the proximal leading edge of the supinator muscle can be identified as a fascial band.

    The PIN is now visible passing distally deep to the SRN and ECRB branch. A yellow sloop has been passed around the complete radial nerve above the PIN take-off to demonstrate the anatomy and show how the PIN starts to branch prior to reaching this leading fascial edge of the muscle. The medial and lateral supinator branches can now be seen separating from the main PIN.

    The fascial leading edge of the supinator muscle can be carefully released with scissors. Do not release it completely at this point until the supinator branches have been identified and protected with a sloop. Releasing 2mm of the fascial leading edge creates sufficient mobility in the PIN to lift it gently from its bed exposing the supinator branches.

    The mixter is passed under the supinator branches separating them from the rest of the PIN.

    Operation – 2

    The red sloop is being passed under the two supinator branches of the PIN.

    The supinator branches are now protected and separate from the rest of the PIN.

    The rest of the superficial head of the supinator can now be released to expose the supinator branches entering the underside of the superficial head.

    Separation of the cut proximal supinator allows further exposure of the distal PIN and the supinator branches. This additional length gained is essential for nerve transfer as distally as possible into the PIN.

    The relevant anatomy is now fully exposed. This demonstrates the typical arrangement seen. The ECRB branch can arise directly from the radial nerve or from the PIN.
    Yellow: PIN
    Red: Supinator branches x2
    Blue: SRN and ECRB branch

    Nerve stimulation of the supinator branches at 0.1mA shows normal contraction of the supinator muscle (C6).

    Direct stimulation of the PIN shows no finger extension from 0.1-5mA. In this case there was a degenerate C8 root compression and some preservation of EPL either due to an incomplete C8 lesion or due to predominant C7 innervation. The possibility of an aberrant innervation should also be considered.

    At 0.5mA the topography of the PIN is mapped identifying that the intact function to EPL is in the superficial part of the PIN at this point. This component of the PIN will be left intact and the supinator branch transfer will be to the non-functioning C8 component to the fingers and ECU.

    The hand and wrist should be carefully observed during stimulation to ascertain the nerve topography and identify the fascicle with preserved function to EPL.

    The PIN is opened longitudinally using jewellers forceps and the curved serrated nerve scissors under the operating microscope. A microsurgery background material is used to aid visualisation of the small nerve branches.

    The fascicle structure is examined.

    Nerve stimulation is used with increasing thresholds on each fascicle group whilst observing the hand and wrist to identify the fascicle group carrying preserved function to the EPL. This group will be spared from the transfer.

    The two supinator branches are sectioned at the muscle and elevated as a common nerve trunk proximally in readiness for the nerve transfer. The donor nerves are always sectioned distally to allow sufficient length for transfer and a tension-free co-aptation.

    The “live” supinator branches are now available for transfer to the PIN.

    The operating microscope is essential for this intraneural dissection and for the co-aptation. The microscope is positioned to afford access to both the operator ( seen here in the axilla) and the assistant (at the head end). The scope has a monitor so that the circulating team and the scrub nurse can watch progress and anticipate equipment needs.

    The fascicle group that is the target for the nerve transfer is identified and mobilised using intrafascicular dissection.

    The recipient fascicles of the PIN are sectioned proximally to allow tension-free co-aptation.

    The supinator branches are trimmed and any loose connective tissue and epineurium is removed. They are transposed to the partially sectioned PIN demonstrating a good size match for half the PIN.

    The co-aptation is completed with 4 9’0 nylon sutures, two to each fascicle.

    Stimulation of the intact fascicle demonstrates preservation of the EPL response. Note that stimulating distal to the co-aptation in this way is technically incorrect. If the EPL fascicle had been sectioned and then the transfer completed, stimulation of the nerve distal to sectioning would still provide a response at the muscle because Wallerian degeneration hasn’t completed. Here direct stimulation through an intact fascicle to a sectioned fascice could result in a false positive response.

    Stimulation proximal to the co-aptation confirms that there is intact function in the EPL across the fascicle no included in the co-aptation.

    The completed supinator to hemi-PIN nerve transfer is demonstrated here on the microsurgery background. the yellow sloop can now be carefully removed.

    Tisseel (Baxter) fibrin glue is placed around the nerve co-aptation site and the microsurgical background can be rolled up around the PIN and the nerve transfer to ensure that the whole circumference of the co-aptation is supported with the fibrin glue.

    Completing the fibrin glue application.

    Using DeBakey forceps the microsurgical background is rolled to support the co-aptation as the fibrin glue activates.

    Operation – 3

    Additional fibrin glue is placed around the co-aptation and the cut supinator muscle to provide support and reduce the risk of nerve displacement in the immediate post-operative period. the fibrin glue will only maintain strength for a few days and will be absorbed in two weeks. the co-aptation is performed in a tension-free manner with the arm abducted at the shoulder, extended at the elbow and full forearm supination. This puts the nerves under maximum stretch, however there should be no tension at the co-aptation site due to the donor-distal and recipient-proximal sectioning of the nerves for the transfer.

    The wound is closed in layers with a subcuticular 4’0 Monacril suture to the skin.

    The final wound closure. if the operation is under general anaesthesia local anaesthetic infiltration to the wound edges will help with early post-operative pain. It is important not to infiltrate deeply as in such circumstances it is reassuring to assess the preserved EPL function with the patient in the recovery room. No local anaesthesia is required if there is a long acting regional anaesthetic block in place.

    Steristrips applied to support the wound edges.

    An occlusive dressing is applied.

    A bulky wool and crepe bandage supports the arm at the operation site and provides light pressure to reduce the risk of a post-operative operation site haematoma.

    Post Operative

    This procedure may be performed in combination with the brachial to anterior interosseous nerve (BrAIN) transfer for restoration of digital flexion and extension in an isolated complete C8,T1 root avulsion injury. The BrAIN procedure will be featured elsewhere in OrthOracle.
    A waterproof dressing with absorbent pad is applied to the wound. The wound should be kept clean and dry for 5 days after which the patient can shower then replace the dressing. A polysling is applied to the upper limb with a torso strap around the waist before the patient wakes from surgery. The polysling and torso strap should be maintained for 3 weeks to prevent excessive passive movement at the elbow
    During this phase the patient is encouraged to maintain isometric contraction of the donor muscle (supinator) in the sling and to visualise the combination of tsupination and digit extension action.
    Nerve transfer rehabilitation involves a 6-phase programme of activity developed at the Centre for Nerve Injury and Paralysis, Birmingham, UK.
    Phase 1 – Pre-operative phase: Education and donor optimisation. Introduction to trophic stimulation and the concept for functional electrical stimulation (FES).
    Phase 2 – Protection phase: During the immediate post-operative period the nerve transfer is protected from inadvertent injury with the arm immobilised. Isometric contraction of the donor and visualisation of the combination donor-recipient action is performed during this period which typically lasts 3 weeks.
    Phase 3 – Prevention phase: During this phase the arm is mobilised and neural gliding is commenced. Joint range of motion exercises (active and passive) are commenced to prevent joint contractures developing. The isometric exercises are continued and isotonic and eccentric exercises are commenced for the donor muscle to maintain function and restore strength. Functional stimulation can be commenced on the donor muscle. Trophic stimulation can be maintained on the recipient muscle.
    Phase 4 – Power phase: During this period the donor muscle is strengthened and the recipient muscle starts to respond. Typically the first sign of reinnervation is a tender muscle squeeze sign due to small fibre reinnervation. Typically this is 3 months following transfer but is affected by the distance of the co-aptation from the recipient motor point.Visible flickers of contraction follow within 6 weeks and donor activation potentiate the recipient response. FES continues and the phase lasts for approximately 6-12 months during which useful motor grade returns: Medical Research Council – (MRC) Grade 3-4.
    Phase 5 – Plasticity: During this phase the patient works on activation of the recipient muscle without activation of the donor. This phase can overlap with phase 4 and is guided by a therapist specialised in nerve transfer rehabilitation.
    Phase 6 – Purpose: During this phase the patient introduces function tasks discussed as objectives during the pre-operative phase. This period of training is tailored to the individual and includes work hardening. Improvements are typically found in power and functional performance for at least 2 years following nerve transfer surgery.

    Recommended Reading

    Outcomes assessment can be with MRC scales but because of the huge functional variation within MRC Grade 4, I prefer absolute muscle testing with digital myometry and comparison with the contralateral limb. In addition fatigue testing and functional scores are important. The EQ5D, DASH and the BrAT scores are in common use in assessing upper limb function after brachial plexus injury. The Canadian Occupational Performance Measure (COPM) is an ideally suited tool to assess patient specific objectives and outcomes.
    Author’s note:
    This transfer is technically demanding and the decision making is complex. In this case there was partial preservation of PIN with intact EPL function and this fascicle group was preserved. In a complete C8,T1 root avulsion the PIN will have a LMN lesion and transfer will be to the whole PIN. In a tetraplegic UMN lesion the PIN will stimulate but careful pre-operative evaluation will have demonstrated no intact volitional function in the PIN and the procedure would be completed with transfer to the whole PIN.
    References:
    Nerve transfers for the restoration of function after spinal cord injury. Mackinnon S, Yee A, Ray W. J Neurosurg.2012; 117: 176-185
    Nerve transfers to retore upper extremity function in cervical spinal cord injury: Update and preliminary outcomes. Fox I et al. PRS. 2015; 4: 780-792

    Reference

    • orthoracle.com

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