Muscle strength measurements of the Hand - Handen Team Zeeland

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Ton A.R. Schreuders, JW Brandsma, HJ Stam FIGURE 5. Prevention of IP flexion contractures by massaging the fingers. Exercises to strengthen the interossei muscles are all aimed at movements which flex the MCP and extend the IP joints. Therefore, exercises for the interossei muscles are all activities in intrinsic plus position: e.g. grasping a book, plate or a cylindrical object like a large bottle. Specific training of the first dorsal and palmar interossei are activities for which key and tip pinch activities are required. To correct the long flexor tightness, the patient is taught to stretch the flexors by holding the hand flat on e.g. a table and by moving the forearm towards an angle perpendicular to the table. In a similar fashion, the hand can be placed on the seat of the chair while the patient sits on the hand and pulls the forearm towards the body. A night splint with the fingers and hand in extension might be necessary in severe cases. 1.2 Lumbricals 1.2.1 Functional anatomy The lumbrical muscles are unique muscles in several aspects. They connect two extrinsic antagonistic muscles. Proximally the lumbricals are attached to the FDP and distally they are inserted into the lateral band of the extensor tendon. The third and fourth lumbricals also connect, by their bi-penal origin, two adjacent FDP tendons. The function of the lumbrical muscles is much debated and some even considered these muscles to be redundant. Brand suggested that the lumbrical muscles are not relevant for MCP flexion. He explained this with an illustration of a father carrying a child; it does not matter what the child (i.e. lumbrical) is carrying, the father (i.e. FDP) has to carry it anyway. 4 Therefore, the lumbrical muscles have a unique ability to contract without adding flexion torque at the MCP joint, in contrast with the interosseous muscles which, when extending the IPs, need a stronger contraction of the EDC to counteract the flexion moment at the MCP joint level. The lumbricals provide a more efficient source for IP extension than the interossei. Any contraction of the lumbrical muscle for IP extension simultaneously, reduces the visco-elastic force of the FDP tending to flex the IP joints. Accordingly the lumbrical can be regarded as a deflexor of the PIP joint. 57 Its direct contribution to MCP flexion is small and in the flexed finger may be non-existent, but its indirect contribution to IP joint extension by decreasing the flexion torque is quite substantial. 58 With the smallest physiological cross-sectional area, it is certainly not a strong muscle. The lumbricals have a very long fiber length (40-48 mm) which indicates that they are designed for long excursions. If the lumbrical fiber length was short, FDP excursion could stretch the lumbrical sarcomeres to a point that they were unable to generate active force. 33 The lumbrical muscles are richly endowed with muscle spindles, their passive stretch by contraction of the FDP might both inhibit finger extensors and facilitate wrist extensors. 58-61 For this reason the lumbrical muscles have been called “tensiometers” between long flexors and extensors. 62 Leijnse and Kalker 46 concluded that the lumbricals are in an optimal position for proprioceptic feedback concerning the PIP-DIP joint mechanism. These unique properties of the lumbricals indicate that they are probably important in fast, alternating movements, e.g. in typing and playing musical instruments. 63 10
Muscle strength measurements of the Hand 1.2.2 Pathokinesiology (paralyses, consequences for prehension/ADL, shortening) In low median nerve injuries the lumbrical muscles of the index and middle finger are paralysed. In these hands it is difficult to discover any problems in the motion of these fingers. 64 A mildly diminished extension of the DIP joint has been noticed in a few patients, which might be explained by the decreased extension force on the extensor apparatus. The “lumbrical plus” is a situation in which there is a FDP tendon rupture distal of the lumbrical origin, or in the situation where a too long graft has been used. The FDP now pulls through the lumbrical muscle rather than through its tendon, causing PIP extension. 65 1.2.3 Assessment possibilities (manual and instrumental) Manual muscle strength testing (MMST) of the lumbricals is practically impossible because of the synergistic action of the interossei muscles. Strength testing is most likely less relevant than evaluating the co-ordination and dexterity in e.g. a tapping test when the lumbrical is paralysed and normal function. However, no studies have been found in which this has been utilised. The strength can be measured in isolation of the interossei muscles in patients with an ulnar nerve lesion where the index and long fingers have only the lumbrical to maintain the intrinsic plus position. One study measured a mean MCP joint flexion strength in the index and long finger of 0.8 kg (range 0.3-1.5) compared with 6.4 kg (range 4.6-7.9) in the non-involved hand. Thus, the affected fingers have only about 12% of the strength of those of the non-involved hand. 64 1.2.4. Therapy principles (prevention complications, strengthening, ADL) No specific training/splinting program for lumbrical paralysis has been advocated. Strength training is similar to interossei muscle training, with perhaps more focus on speed and co-ordination. 2. Thenar muscles The median nerve innervates the intrinsic thumb muscles that make the hand a “human” hand. These muscles oppose the thumb to the fingers: abductor pollicis brevis (APB), opponens pollicis (OP) and the flexor pollicis brevis (FPB). All these muscles originate from the flexor retinaculum and the trapezium carpal bone. Located in the thumb web the adductor pollicis (AP) is sometimes also considered a thenar muscle and will be discussed separately. 2.1 Abductor Pollicis Brevis (APB) 2.1.1 Functional anatomy The insertion of the APB is at the radial side of the proximal phalanx of the thumb. Even though the APB is the smallest intrinsic thenar muscle, atrophy is quickly noticed as it is the most superficial muscle. The main function of the APB is moving the thumb away from the palm, in a perpendicular direction to the palm of the hand, when grasping objects. This is usually called palmar abduction of thumb, but in more recent terminology anteposition of the thumb. 66 The muscle is relatively weak as the action of abduction of the thumb is not one that is usually done against resistance; the APB “only” positions the thumb for action. This movement in the carpometacarpophalangeal (CMC) joint of the thumb is a synergistic action with the OP and, especially, the FPB. 67 An extrinsic synergist of the APB is the abductor pollicis longus (APL). The pull of the APL on the CMC joint of the thumb causes palmar abduction, especially when the wrist is in flexion. Flexion of the wrist increases the moment arm of the APL due to bow stringing. 4 Since the APB has a dual insertion into the base of the proximal phalanx and into the extensor tendon expansion, it also has an extension moment on the IP joint. After loss of the extensor pollicis longus (EPL) the APB together with the oblique head of the AP may provide complete extension of the IP joint of the thumb. This is clinically important when evaluating 11
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Muscle strength measurements of the Hand - Handen Team Zeeland

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