"Intrinsic Plus" is a condition caused by tight interossei and lumbricals, leading to MCPJ hyperflexion and PIPJ hyperextension, managed through therapies or surgical procedures like intrinsic release to alleviate symptoms and restore hand function.
Intrinsic Plus condition is marked by tight interossei and lumbricals muscles causing MCPJ hyperflexion and PIPJ hyperextension, initially identified by Finochietto in 1920 and elaborated by Bunnell in 1946.
Interossei and lumbricals contribute to MCPJ flexion, IPJ extension, grip, and pinch strength in hands.
Intrinsic tightness can stem from local issues like fractures or systemic conditions like Rheumatoid Arthritis, affecting the interosseous muscles.
Diagnosis involves identifying reduced PIPJ flexion in MCPJ extension (Bunnell's Test), with severe cases leading to non-correctible contractures and swan-neck deformities.
Non-surgical treatment aims to prevent irreversible contractures, employing methods like elevation, splinting, and manual stretching depending on the joints involved.
Surgical options vary based on severity and affected joint, ranging from distal/proximal releases to muscle slides or ulnar neurectomy, targeting the restoration of hand function and correction of deformities.
Definition of Intrinsic Plus
"Intrinsic Plus" is the tightness of interossei & lumbricals causing MCPJ hyperflexion and PIPJ hyperextension, first noted by Finochietto in 1920 and detailed by Bunnell in 1946.
"Intrinsic Plus" is the tightness of the interossei & lumbricals. This results in an imbalance of forces that presents as MCPJ hyperflexion and PIPJ hyperextension. Chronic tightness can result in contractures and swan-neck deformities.
Intrinsic tightness is passively correctable. Intrinsic contracture is not passively correctable.
Intrinsic Muscle Anatomy
Interossei and lumbricals muscles provide MCPJ flexion and IPJ extension. They also contribute to grip strength and pinch strength.
An intrinsic muscle is defined as those having an origin and insertion within the hand. This includes the thenar and hypothenar muscles, interossei, and lumbricals. In general, the fingers are controlled by 6 muscles
- 3 intrinsic: dorsal and palmar interossei, lumbrical
- 3 extrinsic: FDS, FDP and EDC (index & little have an extra extensor)
The intrinsic muscles produce MCPJ flexion and IPJ extension because the muscles travel volar to the axis of rotation of the MCPJ but dorsal to the axis of rotation of the IPJ.
The fingers are stabilised by 4 dorsal & 3 volar interossei. These muscles' primary functions are:
- MCP flexion/stabilization and IPJ extension.
- Palmar interossei also adduct the finger
- Dorsal interossei also abduct the finger (Lauer 1999)
Please note that the little finger has a slight variation in horizontal movements. The little finger is abducted by the extensor digiti quinti and abductor digiti quinti.
These muscles originate on the side of the metacarpals. Their insertion points differ. Volar interossei insert onto the index, ring, and small fingers. Whereas the dorsal interossei insert onto the index, middle and ring. More specifically:
- 1st interosseous inserts onto the index finger.
- 2nd and 3rd interossei insert onto the middle finger
- 4th interosseous inserts onto the ring finger.
Interossei insertion can vary. 1st insertions to proximal phalanx, extensors and volar plate, essential for pinching. 2nd-4th also insert into the lateral band. Volar interossei have no bony attachments.
There are 4 lumbricals in the hand innervated by the median (radial lumbricals) and ulnar (ulnar lumbricals) nerves.
- 1st & 2nd lumbricals originate from the radial side of index & middle FDP, insert onto the dorsal expansion as the radial lateral band.
- 3rd & 4th lumbricals are bipennate & originate from the middle/ring and ring/little finger FDP.
Intrinsic Plus Aetiology
Intrinsic tightness, shortening of interosseous muscles, arises from local issues (e.g., fractures) or systemic conditions (e.g., Rheumatoid Arthritis).
Intrinsic tightness/contracture is the shortening of the interosseous muscles. It can be the result of ischaemia, adhesions, displacement, or spasticity. These pathophysiological states are associated with localised or systemic conditions:
- Localised: fractures, infection, burns, compartment syndrome, lumbrical-plus
- Systemic: Rheumatoid Arthritis, spasticity
- Metacarpal fractures can result in oedema, immobilization, adhesions and ultimately fibrosis of the tendons and muscle bellies (Paksima, 2012)
- Proximal phalanx fractures can displace the lateral bands and stretch with callous formation. Adhesions may also form within the lateral bands, which would reduce PIP flexion regardless of MCPJ position (Paksima, 2012). This is why some extra-articular proximal phalanx fractures can still affect the PIPJ movement post-operatively.
- Redirects intrinsic force solely to the PIP joint
- Can progress to swan neck deformity and intensifying intrinsic contracture due to volar plate attrition (Schweitzer, 2004)).
Lumbrical Plus Finger
- Cause: FDP injury distal to the origin of lumbrical, lax tendon graft, distal amputation, high median nerve palsy.
- Effect: FDP (lumbrical origin) retracts proximally, increases tension on the lumbrical & its insertion (lateral band) (Parkes, 1971)
- Result: "paradoxical extension" of the PIPJ with flexion/difficult PIPJ flexion.
- These intrinsic tightness types result from adhesions, inflammation & potentially hypoxia (Akhavani, 2011).
- Rheumatoid arthritis can also cause intrinsic contractures due to MCPJ dislocations and ulnar deviation (Heywood 1979)
- This can be related to cerebral palsy, stroke, tetraplegia, or traumatic brain injuries. It can be associated with shoulder & elbow contractures.
Intrinsic Plus Clinical Picture
"Intrinsic plus" hand is clinically diagnosed through reduced PIPJ flexion in MPCJ extension (Bunnel's Test). Severe cases can cause non-correctible contractures and swan-neck deformities.
An "intrinsic plus" hand is a clinical diagnosis that can be supported by specific investigations to identify the aetiology. It is important to note that intrinsic tightness describes a less severe process - PIPJ flexion deficiency is still passively correctable.
Loss of grip strength
Patients may complain of difficulty with grasping large cylindrical objects. This is because of a restricted PIPJ when the MCP is extended. More specifically:
- Normal Intrinsics: when the fingers grasp around a large sphere, the FDP, FDS and intrinsic simultaneously flex the DIPJ, PIPJ and MCPJ, respectively
- Abnormal Intrinsics: IPJs flex before the MCPJs to produce a hook fist. There is also limited PIPJ flexion when MCPJ is extended.
Patients are often still able to flex the IP joints due to the flexor tendons ability to overpower the intrinsic.
The intrinsic tightness test was developed by Bunnel in 1946.
- Step 1: passively extend MCPJ (stretch intrinsics), measure PIPJ flexion.
- Step 2: passively flex the MCPJ (relaxes intrinsics) and measure PIPJ flexion
- Result: a PIPJ flexion increase suggests isolated intrinsic tightness.
This test can be also performed in radial or ulnar deviation. For example, placing the MCPJ into radial deviation during the test would further tighten the ulnar-side intrinsic tendons.
Swan neck deformity
A swan neck deformity can occur in severe chronic contractures by the following mechanism
- Cause: Progressive PIPJ contracture leads to hyperextension.
- Effect: Terminal tendon slackens while the FDP tendon tightens.
- Result: DIPJ flexion ensues, leading to a swan neck deformity. Over time this can result in volar plate attenuation or collateral ligament capsular contraction
An intrinsic-plus hand is primarily a clinical diagnosis. Adjunctive investigations can be considered, such as:
- Isolated strength & pinch measurements
- X-rays to assess subluxation in rheumatoid patients
- Rheumatoid serology testing.
- Nerve block in spasticity before the intrinsic tightness test can assist the examiner in discerning increased muscle tone from fixed contractures
- Dynamic electromyography (EMG) to evaluate the level of volitional control
Non-Surgical Management of the Intrinsic Plus Hand
Treatment for finger intrinsic contractures depends on the cause. Determine whether it involves just the PIPJ or both PIPJ and MCPJ to guide management.
The primary goal of treatment is to prevent a contracture that is no longer passively correctable. Non-surgical treatment should be continued until progress is no longer observed. Options include:
- Elevation to reduce oedema, serum protein deposition and scar tissue formation.
- Splinting in the intrinsic plus position (MCPJ flexion 70°, IPJs in extension) to optimise the MCPJ collateral ligaments & keeps intrinsics in a shortened position.
- Manual stretching of the intrinsics by passive abduction or by MCPJ extension and IPJ flexion (Seu 2012)
- Progressive static splints hold the MCP joints in extension and allow passive hook exercises (ie: perform the intrinsic tightness test)
Surgical Management of the Intrinsic Plus Hand
Mild intrinsic tightness can be treated with hand therapy or distal release. Severe disease requires correction of swan neck deformity. Nonfunctioning intrinsics may be completely released or denervated
A variety of procedures are available to the intrinsic-plus patient. The choice is dependent on the aetiology, severity and most importantly the joint involved. Options include proximal and distal releases, slides or translocations
Distal Intrinsic Release for PIPJ
- Indication: Consider mild cases to improve PIPJ motion when patients struggle to make a full fist or weakness of grip.
- Purpose: decrease the intrinsic force on the PIPJ without affecting the intrinsic flexion of the MCPJ.
- Technique: resection of the ulnar and/or radial lateral bands & oblique fibres in the distal third of the proximal phalanx. This is distal to the transverse fibres that are responsible for MCPJ flexion. If the release is inadequate, then progressively more volar and proximal tissue is released until passive PIPJ flexion is achieved (Espiritu, 2011)
MCPJ Flexion Treatment
MCP flexion contractures secondary to intrinsic tightness are treated based on severity and aetiology. Options include a proximal intrinsic release (PIR), intrinsic muscle slide, botox injections, or ulnar nerve neurectomy.
Proximal Intrinsic Release
- Indication: MCPJ flexion contracture when intrinsic muscles have become fibrotic and no intrinsic muscle function is anticipated:
- Technique: divide radial and ulnar interosseous tendons proximal to the MCPJ. The sagittal hood fibres of the extrinsic extensor are identified and preserved while the transverse and oblique fibres of the intrinsic mechanism are released proximal to the MCP joint. The transverse portion of the intrinsic flexes the MCPJ.
Interosseous Muscle Slide/Fractional Lengthening
- Indication: residual intrinsic function from ischemic or spasticity.
- Technique: Elevate periosteal origins of dorsal and volar interossei from metacarpals ± first dorsal interosseous attachment to the thumb metacarpal. Splint MCP joints in full extension to allow the origin to heal distally.
- Tip: fractional lengthening is an alternative option. Obliquely incise the tendinous part of the myotendinous junction, preserving the muscular portion.
- Indication: spastic patients without volitional control
- Technique: Excise the ulnar nerve's motor branch in the Guyon canal. Index/middle lumbricals remain slightly active.
- Note: In pronounced spasticity, extrinsics can overshadow intrinsic tightness. Post-extrinsic release, if MCP extends passively, neurectomy halts intrinsic contracture. If not, tenotomy near MCP aids extension
Chronic intrinsic tightness results a swan neck deformity with volar plate laxity.
- Indication: swan neck deformity due to chronic PIPJ hyperextension.
- Technique: Distal release of tighter intrinsic, reroute contralateral intrinsic tendon beneath the Cleland ligament to counteract hyperextension. Secure the tendon to the proximal phalanx after tenotomizing at its proximal third.
- Tip: a lateral band translocation can be performed if concerns regarding Cleland ligament's strength in holding volarly-positioned lateral bands. Divide Cleland, locate the volar plate and flexor sheath, create a dorsal flap, translocate the lateral band volar, and reposition the flap anatomically.
Lateral Band Mobilisation
- Indication: Rigid swan neck deformity
- Technique: extensor tenolysis, dorsal capsulectomy, and lateral band mobilization via a dorsal incision. Release lateral bands from central slip and triangular ligament.
Radial Lateral Band Resection
- Indication: lumbrical plus finger resulting from lumbrical tightness.
- Technique: Transect radial lateral band at the origin (palm) or insertion (dorsal apparatus) to mitigate paradoxical extension.