//

Median nerve- Proximal release at the pronator tunnel

    Overview

    Learn the Median nerve: Proximal release at the pronator tunnel surgical technique with step by step instructions on OrthOracle. Our e-learning platform contains high resolution images and a certified CME of the Median nerve: Proximal release at the pronator tunnel surgical procedure.
    The commonest site of compression of the median nerve is the carpal tunnel. Rarely, a primary more proximal compression may be responsible for symptoms and a careful review of patients with failed symptom resolution after carpal tunnel decompression (CTD) may identify a source of proximal compression that requires release.
    There are various common sites pression at this site may be due to the lacertus fibrosis, the proximal edge of the pronator muscle or the flexor digitorum superficialis (FDS) arch. The typical symptoms include proximal forearm pain, sensory disturbance in the radial volar digits and palm, Tinel’s sign at the point of compression and pain on resisted pronation or resisted FDS contraction to the middle finger.
    Imaging of the nerve should be performed prior to surgery to exclude an intrinsic nerve tumour or extra-neural mass causing pressure in this tight space. Surgery aims to identify the median nerve at the proximal edge of the lacertus fibrosus and trace distally decompressing the nerve throughout its course while protecting proximal motor branches.
    Dominic Power MA MB BChir(Cantab)FRCSEd FRCSLon FRCS(Tr & Orth)
    Consultant Hand and Peripheral Nerve Surgeon
    Honorary Senior Clinical Lecturer, University of Birmingham, UK
    West Midlands Peripheral Nerve Injury Service
    Birmingham Hand Centre, UK


    Indications

    INDICATIONS

    -Proximal median nerve compression may occur as a primary compression neuropathy or secondary to nerve compression due to intrinsic or extrinsic tumours.
    -Recovering peripheral nerve injuries may auto compress at naturally tight anatomical locations because of the diameter of the growth cone and interrupted axoplasmic transport.
    -Surgery may be required in case of failed CTD when there is clinical evidence to support a proximal compression.
    -Spontaneous onset anterior interosseous syndrome is a primary motor neuropathy of uncertain aetiology. It may be related to a nerve compression or related to a post-viral syndrome.
    In such cases pain with clinical evidence of irritation at the proximal forearm and a positive Tinel’s sign, patients may benefit from exploration and decompression of the proximal median nerve which is aimed at preservation of function or recovery of function if there is a compressive element. The decompression should always encompass all potential sites of entrapment from the lacertus fibrosus, through pronator teres to the FDS arch.
    SYMPTOMS & EXAMINATION
    In a proximal median nerve compression, patients will report proximal and mid forearm pain and sensory disturbance that affects the whole of the radial palmar surface of the hand including the median and palmar branch median nerve territories. In severe compression there may be weakness of median innervated muscles including FDS, FDP to the index and middle fingers and FPL. Pronator weakness may be apparent from combined involvement of pronator teres and pronator quadratus, although typically there is pronator teres preservation unless the entrapment is more proximal involving a ligament of Struthers in the lower median arm or a tight lacertus fibrosus. The ligament of Struthers is thought to be a developmental anomaly and perhaps the remnant of an accessory coracobrachialis head that incompletely forms and may form a rigid band or even an ossified tunnel over the neuromuscular bundle. Severe compression will also be associated with thenar muscle wasting and weakness of palmar abduction and opposition.
    Clinical examination may demonstrate pain and sensory symptoms reproduction on resisted middle finger FDS contraction (performed with the other fingers extended to prevent FDP recruitment). Resisted pronation of the forearm with the elbow flexed may produce median nerve symptoms if the compression is at the proximal protantor teres. Tinel’s sign will be positive at the FDS proximal fibrous arch. Opposition strength should be assessed together with abductor pollicis brevis strength in the hand. Quantitative sensory testing can be performed using static or moving two-point discrimination and monofilament pressure threshold detection. In severe cases with anterior interosseous nerve involvement there will be reduced strength of FPL and FDP to the radial two fingers. The “OK” sign made between the thumb and index is not possible with severe weakness and in such cases the attempt produces an incomplete circle with IPJ extension of the the thumb and DIPJ extension of the index finger.
    IMAGING
    Plain radiographs are useful in excluding an ossified ligament of Struthers. Ultrasound is useful in tracing the nerve and looking for changes in diameter that may result from compression. MRI is the most useful examination for excluding tumours within the nerve or extrinsic tumours compressing the nerve.
    Neurophysiology studies are useful to exclude a distal compression at the carpal tunnel. Electromyography (EMG) is useful to demonstrate muscle denervation with fibrillation and positive sharp waves evident with acute denervation. Polyphasia on EMG is a sign reinnervation and may be present in longstanding incomplete compression or a partially recovered anterior interosseous syndrome.
    ALTERNATIVE OPERATIVE TREATMENT
    Surgery may be targeted at the pronator and distally to the FDS arch in cases with symptoms at the proximal forearm and more proximally with symptoms and a Tinel’s sign at the level of the ligament of Struthers. In all cases `I decompress the lacertus fibrosus that may contribute to the compression. In cases where there is severe and longstanding motor loss in the anterior interosseous nerve territory, an alternative strategy of motor functional restoration using tendon transfers with decompression only for pain and improving sensory symptoms. The commonly described tendon transfers include brachioradialis transfer to the FPL, buddying of the median denervated FDP tendons to the ulnar innervated FDPs in the distal forearm and an opposition reconstruction, typically using the extensor indices transfer to abductor pollicis brevis tendon using a pulley re-routing through a distally based loop of the flexor carpi ulnaris tendon.
    NON-OPERATIVE MANAGEMENT
    In mild cases, an expectant policy can be adopted. When there is demonstrable motor weakness, I would recommend a surgical approach.
    CONTRAINDICATIONS
    There are no absolute contraindications to proximal median nerve decompression. Some recommend tendon transfer alone in complete motor loss and consider the causation a post-viral neuropathy that is not salvageable. However, in my experience there is always an element of uncertainty regarding causation and there is always potential for some recovery in cases of compression. I would decompress all those with some pain in the proximal foam and clinical evidence of local irritation at this site. Intra-operative nerve stimulation can demonstrate whether there is any retained function in the motor component of the proximal median nerve and its anterior interosseous component. Improvements in stimulation thresholds are a useful indicator of post-operative improvement potential.

    Setup

    Patients are consented for exploration and decompression in the proximal forearm. The surgical site is marked. Anaesthesia may be general or regional with ultrasound and nerve stimulation blockade at the axillary level and upper medial arm. Distal top-ups in cases of incomplete blocks are to be discouraged as local anaesthetic in the region of surgical decompression will interfere with nerve stimulation intra-operatively.
    Basic hand instruments with Jamieson scissors and DeBakey forceps are useful for the deep dissection around the nerve. A mister forceps with a 90 degree tip is useful for passing surgical loops around the median nerve and its branches. Self-retaining West and Travers retractors are required to help with the deep decompression. A nerve stimulator box and stimulating needle are useful with an arthroscopy camera drape cover to allow the needle to be kept sterile while an unscrewed member of the team can control the stimulation thresholds.
    A tourniquet placed around the upper arm is recommended to ensure a bloodless field.
    The limb is draped with the hand exposed to allow assessment of hand function during stimulation.

    Operation – 1

    The hand is positioned in a lead hand to control forearm rotation.
    The surgical sites are marked. In this case there was an infraclavicular injury to the brachial plexus with axonopathy and good evidence of regeneration with progressive Tinel’s sign. There remained on repeated clinical examinations (3 months apart) strong Tinel’s signs at the pronator tunnel and in both the carpal tunnel and Guyon’s canal.
    Both of these distal compression sites will be decompressed through a single incision. Carpal tunnel decompression https://www.orthoracle.com/library/carpal-tunnel-decompression/and Guyon’s canal decompression https://www.orthoracle.com/library/guyons-canal-release/are covered as separate technique descriptions in OrthOracle.
    This technique will concentrate on the surgical approach for proximal decompression of the median nerve at the level of the pronator tunnel.

    The skin is incised in the proximal third of the forearm after the tourniquet is inflated following limb exsanguination with an Esmarch bandageThe incision starts proximally on the medial side of the biceps tendon over the neuromuscular bundle. The incision moves radially towards the pronator insertion and then swings back towards the midline of the forearm. This will allow access to the pronator teres tendon for lengthening, to facilitate access to the deeper and more distally placed FDS proximal tendinous arch.

    The fat is exposed using blunt dissection.Care is taken to identify and protect any large cutaneous nerve branches. The lateral cutaneous nerve of the forearm is radial to the incision but should be identified and protected.

    The fat is exposed using blunt dissection.Bipolar diathermy is used to coagulate any small crossing vessels. The deep fascia should be exposed.

    A small West self-retaining retractor is placed in the wound edges in the fat layer.The deep fascia is thus exposed.

    The fat layer needs careful dissection to avoid damage to cutaneous innervation.Crossing nerve branches should be identified, mobilised and protected.

    The underlying muscles can now be seen beneath the fat.The fascia is cleared with blunt dissection after removal of the self-retaining retractor.

    The self-retaining retractor is repositioned in the proximal end of the wound.The junction between the pronator muscle on the medial side of the wound and the fat overlying the neurovascular bundle is seen deep to the fascia.

    The fascia is incised at the junction between the muscle and the fat.Using scissors the fascia is split longitudinally in proximal and distal directions. The lacertus fibrosus (LF) is the medial insertion of the fascial condensations arising from the biceps tendon and running obliquely and distally towards the subcutaneous border of the ulna shaft. The fascia is thickened and is sharply incised. It is important to ensure that the division proceeds proximally to the most proximal edge of the lacertus fibrosus.

    The careful use of sharp dissection may be required in releasing the fascia.The scalpel may be used where the lacertus fibrosus is thick and fibrotic.

    Identify the neurovascular bundle deep to the fascia.The flexor-pronator muscle mass is now visible on the ulnar side of the incision and the neurovascular bundle sits just radial to it.
    BA – Brachial Artery
    The median nerve enters the forearm in close proximity to the brachial artery.
    From lateral to medial the structures in the antecubital fossa are the Biceps tendon, the brachial artery (with venae commitantes) and the median nerve.
    The nerve and vessel are covered by the lacertus fibrosus which is the subcutaneous extension of the biceps tendon extending to the subcutaneous border of the proximal ulna shaft.
    The lacertus is a prime elbow flexor and the biceps tendon is a supinator.
    The median nerve has proximal branches to there pronator teres, the flexor carpi radials and the palmaris longus. These fine branches can be injured in exposing the nerve when dissecting in the adipose tissue around the neurovascular bundle.
    The interval between the median nerve and the brachial artery should be developed. Vessels cross the nerve as they course to the flexor-pronator muscle mass and these branches should be identified and divided between ligaclips or following careful bipolar cautery.

    Mobilise the neurovascular bundle (brachial artery, venae commitantes and median nerve).The vascular bundle is seen with the brachial artery (BA) branching to common interosseous artery with venae commitantes. Medial to this the median nerve is exposed with blunt dissection avoiding damage to the proximal branches.

    Mobilise the neurovascular bundle with non toothed forceps.DeBakey forceps are used near the nerve rather than toothed forceps. Bipolar diathermy is used to coagulate small crossing vessels prior to division. The proximal forearm is rich in terms of vascular supply with multiple small vessels. Meticulous haemostasis is essential to prevent post-operative wound complications, bleeding, haematoma formation and compartment syndrome.
    BA – Brachial Artery
    MN – Median Nerve
    The median nerve is located in the fat on the medial side and deep to the brachial artery and the venae commitantes.

    Identify the fascial sling deep to the pronator muscle.There is a thick fascial sling across the nerve deep to the pronator muscle that must be released with scissors.
    Use Jamieson scissors to expose the nerve and separate it carefully from the fascia prior to release. If the area is tight a McDonald’s can be placed under the fascia and superficial to the median nerve to allow safe division.

    Identify small vessels and coagulate and divide each in turn. The vessels block access to the median nerve They must be divided to achieve safe exposure for decompression.
    The vessels can bleed intra-operatively, obscuring the surgical view or post-operatively causing a haematoma and risk of forearm compartment syndrome developing.

    Larger vessels and those adjacent to the fine nerve branches are best divided between ligaclips. This avoids using bipolar diathermy near nerve branches and allows better vascular control than diathermy alone on the larger vessels.

    In the superficial part of the neurovascular bundle the nerve to pronator teres can be identified and should be protected.The distal wound shows the radially placed brachioradialis muscle under which the pronator teres passes at it becomes tendinous. The pronator teres (PT) inserts onto the lateral aspect of the radius shaft at the midpoint between elbow and wrist. The PT action is to pronate the radius.
    The PT tendon is tight and prevents access to the more deeply positioned median nerve. The PT tendon must be released with a “Z” step lengthening to allow distal and ulnar retraction of the PT to expose the median nerve.
    PT – Pronator Teres muscle
    PTI – Pronator Teres Insertion point
    BR – Brachioradialis

    Careful and systematic dissection exposes the vessels and their branches. The brachial artery gives rise to the radial artery which is visible on the ulnar edge of the brachioradialis. The vessels have multiple small branches to the surrounding muscle and the branches should be divided after bipolar cautery to allow safe expose of the median nerve in the deeper layers. Large branches are best divided between ligaclips.

    The pronator tendon should be released.The plane along the radial side of the pronator muscle is developed further distally to follow the pronator muscle.It should be followed to its musculotendinous junction as it passes towards the mid-shaft radius.
    The pronator tendon needs releasing to allow sufficient retraction to expose the deeply placed median nerve and its branches throughout the course of the pronator tunnel.

    A Ragnall retractor under the brachioradialis allows exposure of the neurovascular bundle in the distal wound deep to the pronator teres.
    MN – Median Nerve (in fat)
    BR – Brachioradialis
    PT – Pronator Teres
    RA – Radial Artery

    The musculotendinous junction of the pronator teres is now visible in the distal wound.
    PTT – Pronator Teres Tendon
    The PTT passes distally deep to the brachioradialis and towards the radial border of the mid shaft of the radius into which it inserts. The PTT is covered by the BR muscle and the tendons of the radial wrist extensors (ECRL and ECRB).

    The humeral head of the pronator teres muscle is divided with a long “Z” shaped incision whilst the forearm is supinatedThe humeral head of the pronator teres muscle forms the superficial part of the pronator teres tendon as it passes to the radius shaft.
    This part of the tendon is divided with a long “Z” shaped incision whilst the forearm is supinated. This step allows the PT to be effectively lengthened.
    This allows the tendon to retract and the pronator teres muscle may then be retracted medially and distally to expose the deeper median nerve, its branches and the FDS proximal tendinous arch.
    The median nerve is deeper and more ulnar than the PTT tendon sectioning site and so is not at risk as long as the step lengthening is done from the insertion only so far as the muscle-tendon junction.

    PT – pronator teres tendon after sectioning
    The pronator teres tendon has been “Z” lengthened and this reduces the tension in the muscle allowing muscle retraction.

    The brachioradialis muscle laterally and the pronator teres muscle medially are retracted with deeper placement of the distal self-retaining retractor.
    Crossing vessels are seen now in the proximal part of the wound and these can be divided after ligaclip applications.

    The vessel is divided between the ligaclips.
    The median nerve lies in the deep part of the wound. It is visualised after release of the PT tendon and retraction of the muscle plus division of all crossing vessels which lie superficial to the nerve.

    Operation – 2

    The median nerve lies in the deep part of the wound. It is visualised after release of the PT tendonThe median nerve is being exposed in the proximal wound and a surgical sloop will be placed around the nerve to allow mobilisation and retraction without risk of injury.

    A vascular sloop is placed carefully around the median nerve.A 90 degree tipped mixter forceps is passed deep to the nerve and a yellow sloop is carefully placed in the jaws using DeBakey forceps. It is important to ensure that the mister forceps don’t capture epineurium in the jaws deep to the nerve and that the sloop doesn’t snag on the nerve as it is passed deep to the nerve.

    The sloop is being retrieved deep to the median nerve.

    At this level the sloop will pass around the median nerve and the proximal motor branches. Lifting the sloop will demonstrate the nerve branches and allow further mobilisation.

    Identification of the distal motor branch of the median nerve.The photograph is now taken from the opposite side of the table to allow visualisation of the distal motor branch of the median nerve. For the purposes of orientation, the elbow is on the right of the photograph and the hand on the left with the view from the radial (lateral) side of the forearm.
    The sloop is around the proximal median nerve.
    Distal dissection allows mobilisation of the pronator muscle and tracing further distally will reveal the FDS proximal tendinous arch.

    Division and excision of the flexor digitorum proximal tendinous arch.The pronator is being retracted ulnar wards and distally and the FDS arch is seen crossing the nerve.
    FDS – Flexor digitorum proximal tendinous arch
    MN – Median Nerve
    PPT – Pronator Teres Tenotomy
    I would recommend that the fibrous arch is excised over a width of at least 1cm and over its full length until FDS muscle only is left in contact with the median nerve. This helps to prevent recurrent scar and nerve compression after an incomplete release.
    Caution should be exercised during the segmental excision of the tendon arch to prevent injury to the small FDS motor branches that lie in contact with the arch separate from the main median nerve.

    The median nerve is carefully handled and mobilised to ensure it is adequately released.The forceps are demonstrating where the nerve and its branches were flattened by the FDS proximal fibrous tendinous arch. The median nerve has a proximal branch to pronator teres then a branch to flexor carpi radials and palmar is longus. Next as it passes deep the FDS proximal tendinous arch there are usually two branches to the FDS muscle then deep and lateral to the main median nerve, there is the anterior interosseous branch (AIN).
    The AIN passes to the interosseous membrane and accompanies the anterior interosseous artery supplying in turn the index and middle FDP, the FPL and the PQ muscles. The terminal branches supply proprioception to the wrist joint and distal radio-ulnar joint.
    The median nerve continues under the FDS muscle and becomes superficial between PL and FCR in the distal quarter of the forearm. The median nerve has a palmar cutaneous nerve branch that leaves the radial median nerve 5cm proximal to the wrist crease and crosses the wrist crease at the radial side of the FCR tendon sheath and supplies the cutaneous innervation over the thenar eminence.
    The remainder of the median nerve enters the carpal tunnel and the usual anatomy os for branching to a recurrent motor branch to the thenar muscle and common digital nerves to the radial digits.
    Proximal median compression at the level of the pronator tunnel will cause sensory disturbance to the thumb, middle index and radial ring fingers plus the thenar eminence skin. This may differentiate it from carpal tunnel where there is sparing of the thenar skin.

    Closure of deep layersThe deep layers are opposed with interrupted absorbable sutures. The fascia is not closed.

    Dermal closure.The dermis is closed with interrupted absorbable sutures.

    Skin closureA continuous monaural 4’0 subcuticular suture is used for the skin.

    Steristrips are applied to the skin.Local anaesthetic is placed in the skin unless the surgery is under a regional anaesthetic with long acting local anaesthetic.

    Supportive bandages are applied to the limb.A supportive soft wool and crepe bandage is applied to the limb to protect the site of surgery. In this case the dressing includes the hand because of the concomitant releases in the hand which are not discussed in this operative technique.

    Post Operative

    The patient is provided with a supportive Bradford sling which should be worn until the regional block has worn off. The arm should be elevated at rest to reduce pain and swelling. The patient is encouraged to mobilise their fingers and use the hand for light functional activity as soon as comfortable.
    Nerve scar is a risk after surgery near a nerve and functional use allows gliding of the nerve.
    The bulky dressings can be reduced after 5 days with a waterproof dressing over the wound for 10-12 days. The wound can be left open at this stage and moisturising scar massage helps maturation.
    Motor and sensory quantitative testing should be assessed at 6 weeks and 3 months post-operatively.

    Recommended Reading

    Proximal median nerve entrapment is rare in isolation and most frequently accompanies distal entrapment at the carpal tunnel level. The failure of all symptoms to resolve after carpal tunnel release should prompt a thorough review and consideration of a proximal pathology. I recommend imaging the nerve at the pronator ever to exclude an extrinsic tumour or intrinsic tumour causing compression. The differential diagnosis is for a post-viral neuropathy which typically results in a motor neuropathy with relative sensory sparing. Full decompression of all potential points of entrapment is required for optimal results. The approach is not only useful for decompression, but is also part of the approach for median to radial nerve transfers for high radial nerve palsy or isolated FCR/PL fascicle transfer to the posterior interosseus nerve for a PIN palsy. Familiarity with the approach also is of use for the occasional penetrating wound exploration presenting to hand trauma services.
    References
    Hsiao CW, Shih JT, Hung ST. Concurrent carpal tunnel syndrome and pronator syndrome: A retrospective study of 21 cases. Orthop Traumatol Surg Res. 2017 Feb;103(1):101-103. doi: 10.1016/j.otsr.2016.10.009
    Of 344 cases with median neuropathy, 321 were CTDS alone, 1 pronator alone and 21 concomitant entrapments at both levels. The pathology was confirmed with NCS and EMGs. 71% of patients had symptom resolution. 6 patients (29%) had occasional paraesthesia and pain.
    Gross PT, Tolomeo EA. Proximal median neuropathies. Neurol Clin. 1999 Aug;17(3):425-45
    This review article reports the aetiology with overuse being a common precipitant. Conservative management with rest and activity modification are helpful in mild cases. EMG is a useful confirmatory test and surgery should be reserved for this with moderate or severe symptoms or failing to resolve with conservative management.
    Olehnik WK, Manske PR, Szerzinski J. Median nerve compression in the proximal forearm. J Hand Surg Am. 1994 Jan;19(1):121-6
    39 proximal median nerve decompressions in 36 patients with 19 limbs in 17 patients having had previous CTD. Most patients present with paraesthesia or numbness in the median territory. The most common finding was a positive pronator test. 37/39 has neurophysiology testing with 12 demonstrating an abnormality. The commonest site of compression at operation was noted as FDS. 30 limbs improved or resolved. 14/19 with previous CTS resolved. Resolution was greater in those with normal neurophysiology.
    Swiggett R, Ruby LK. Median nerve compression neuropathy by the lacertus fibrosus: report of three cases. J Hand Surg Am. 1986 Sep;11(5):700-3
    3 patients with spontaneous onset proximal forearm pain and weakness failed conservative management and all resolved with surgical release.
    Guo B, Wang A. Median nerve compression at the fibrous arch of the flexor digitorum superficialis: an anatomic study of the pronator syndrome. Hand (N Y). 2014 Dec;9(4):466-70. doi: 10.1007/s11552-014-9639-5
    38 cadaveric dissections demonstrated 2 FDS arch subtypes -distinct and indistinct fibrous arches. In those with a distinct arch (42%) it measured 16.9mm in length. In those without a distinct arch 2.6cm of muscle splitting was needed to decompress the median nerve. The AIN left the median nerve at the arch or beyond in 47% of cases with 92% exiting the radial side of the median nerve.

    Reference

    • orthoracle.com

    Tags:

    You might be interested in

    Dark mode powered by Night Eye