Anatomical and Biomechanical Principles of Orthotic Intervention


Figure 2-1. Bones of the UE.



Introduction


In order to design an effective and well-fitting orthosis, the practitioner must have a thorough foundation of upper extremity (UE) anatomy, wound healing, and biomechanical principles. More importantly, this requires the ability of the practitioner to apply this knowledge to meet the client’s particular needs. This chapter presents an overview of UE anatomy and wound healing concepts. The anatomical structures specific to each orthosis in this textbook are highlighted in the instructional videos. Readers are encouraged to refresh their knowledge of specific UE anatomical structures, including bones, muscles (including nerve innervations), joints, ligaments, surface landmarks, and digital anatomy, and of the healing time frames of each of these structures.


It is also critical for a practitioner to consider the biomechanical principles surrounding orthotic design when using orthoses to treat a client’s condition. This knowledge will help the practitioner understand how the orthosis will influence anatomical structures involved and assist them in determining appropriate orthosis designs and wearing schedules. Thorough knowledge of biomechanical concepts related to orthotic fabrication will assist the practitioner in designing a comfortable, effective, and cosmetically pleasing orthosis.


Overview of Upper Extremity Anatomy


The anatomical structures of the UE are influenced by wearing any type of orthosis. It is critical for the practitioner to understand what these structures are, where they are located, and how an orthosis affects them.


BONES AND JOINTS


Depending on its purpose, UE orthoses acts on the bones, joints, and soft tissue structures of the elbow, forearm, wrist, hand, thumb, and digits (Figures 2-1 and 2-2). The majority of joints in the UE are synovial joints. This type of joint is the most common in the body and is designed for movement. These joints permit free motion between the bones they join together. There are eight elements found in all synovial joints (Figure 2-3).


The concavity, or curvature, of the metacarpal and carpal bones on the volar aspect of the hand make up the three hand arches: proximal transverse, distal transverse, and longitudinal. The proximal transverse arch forms a rigid arch at the carpometacarpal (CMC) joints of digits II through V, whereas the distal transverse arch and longitudinal arches are more mobile and contribute to mobility and functional posture of the palm and digits (Figure 2-4A). When an orthosis includes the wrist and hand, it is critical for the practitioner to accommodate these arches to allow for optimal support and alignment (Figure 2-4B). The distal transverse arch must not be obstructed in orthoses that do not include the digits to allow for full, unrestricted movement of the fingers (Figure 2-4C). Note the oblique orientation of the distal transverse arch with the hand in a fist, as compared with a more horizontal orientation with the fingers in extension (Figure 2-4D).



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Figure 2-2. Joints of the UE.




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Figure 2-3. Components of a synovial joint.




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Figure 2-4. (A) The three hand arches are outlined here. (B) It is important to observe and accommodate for the three hand arches in orthoses that include the hand and wrist. (C) Note the oblique orientation of the metacarpal heads when the digits are flexed: higher on the radial side, lower on the ulnar side. (D) Note the horizontal orientation of the metacarpal heads when the digits are in extension.


BONY PROMINENCES


Defined, bony prominences are areas where a bone lies immediately below the surface of the skin (Box 2-1 and Figure 2-5). It is critical for the practitioner to locate all of the prominences that will be covered or affected in some way by a UE orthosis. If not addressed properly, the thermoplastic material and straps of the orthosis can apply excessive compressive and shear forces over the prominences and create pressure areas and discomfort for the client. Left unchecked, these excessive forces can lead to tissue breakdown and pain. The practitioner should take care to use highly conforming thermoplastic materials so it conforms well to the bony anatomy, flare the thermoplastic material away from the bony prominence, and consider use of padding on the inside of the orthosis after the area is flared to avoid unwanted pressure.


NERVE INNERVATIONS


An orthosis for the UE can influence virtually all the nerve pathways in the arm, forearm, and hand. Nerve innervation to all UE muscles arises from the brachial plexus (Figure 2-6A through D). The three terminal branches of the brachial plexus form the peripheral nerves that innervate the majority of the muscles of the arm and all muscles of the forearm and hand.


FOREARM AND HAND CONNECTIVE TISSUES


By virtue of design, an orthosis for the UE affects connective tissues in the forearm, wrist, and hand. It is important for the practitioner to understand these structures and how an orthosis influences them.



Box 2-1. Upper Extremity Bony Prominences


Medial epicondyle


Lateral epicondyle


Olecranon


Ulnar head


Ulnar styloid


Metacarpal heads


Pisiform


Scaphoid tubercle


Radial styloid


Dorsal proximal and distal interphalangeal joints


Thumb CMC joint/base of first metacarpal


Thumb metacarpophalangeal and interphalangeal joints (dorsal aspects)


The skin on the volar surface of the hand is distinctly different from the skin on the hand’s dorsal surface. The palmar/volar skin is thick and immobile and serves to stabilize and protect the underlying structures. The palmar skin attaches to the underlying palmar aponeurosis, a thin, strong layer of fascia attached to the palmaris longus tendon that helps to form the palmar skin creases and assists with grasp (Figure 2-7).


The skin on the dorsal aspect of the hand, conversely, is thin and mobile and allows the underlying tendons of the extensor digitorum to glide freely across the metacarpals and the fingers to flex and extend freely (Figure 2-8).


Due to the relative laxity of the dorsal skin, edema tends to accumulate on the dorsal aspect of the hand following injury. The practitioner must take care to accommodate for this edema when fabricating an orthosis that includes the hand and wrist (Figure 2-9).


Upper Extremity Anatomy by Region


ELBOW AND FOREARM


The elbow contributes significantly to UE function and is the key to positioning the hand and wrist in space.


Elbow Joint


The elbow joint is a complex structure that consists of three separate joints—the radiohumeral, ulnohumeral, and proximal radioulnar—contained in a single joint capsule. The radiohumeral joint is formed by the articulation between the capitulum of the distal humerus and the proximal end of the radius, the radial head. The ulnohumeral joint is formed by the trochlea of the distal humerus and the proximal end of the ulna, the trochlear notch (Figure 2-10).



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Figure 2-5. UE bony prominences are important to observe when fabricating an orthosis.


Both the radiohumeral and ulnohumeral joints are highly congruent and act together to form a modified hinge joint where elbow flexion and extension occur. Normal range of motion (ROM) in the elbow is 0 to 150 degrees, and the majority of this movement occurs at the ulnohumeral joint.


Forearm Joints


The proximal radioulnar joint, also located within the elbow capsule, is the articulation between the radial head of the radius and the radial notch of the ulna. This joint is linked directly to the radiohumeral and radioulnar joints by the elbow capsule, but the movement produced differs. This articulation, along with the distal radioulnar joint at the wrist, allows for forearm pronation and supination and does not contribute to elbow motion. Acting together, all three joints allow the hand to be placed in a wide variety of positions and contribute to the stability of the UE during weightbearing activities such as using a walker or cane, shifting one’s weight in a chair, or pushing and pulling activities. Due to the high degree of conformability between each of the three joints and the single joint capsule configuration, injury to these joints and other associated soft tissues can lead to significant loss of joint movement in elbow flexion/extension and/or forearm pronation and supination.



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Figure 2-6. (A) Outline of the brachial plexus and terminal branches. (B) Median nerve, (C) radial nerve, and (D) ulnar nerve. The musculocutaneous nerve is not shown.


With the UE in anatomical position (humeral adduction, elbow extension, forearm supination), the forearm tends to deviate laterally in relationship to the humerus. This lateral deviation is termed the carrying angle or valgus of the elbow, and it is normally between 10 and 15 degrees from the longitudinal axis of the distal humerus and the mid-forearm between the radius and ulna. This angle is attributed to the medial and distal orientation of the trochlea of the humerus in relation to the capitulum (Figure 2-11).



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Figure 2-7. The palmar aponeurosis is a thick, unyielding structure that helps to form the skin creases in the palm and assists with grasp.




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Figure 2-8. The thin, mobile skin on the dorsal aspect of the hand allows the extensor tendons to move and glide freely during digit movement.




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Figure 2-9. Edema tends to accumulate on the dorsal aspect of the hand due to the laxity of the skin. It is important to accommodate for this when fabricating an orthosis.




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Figure 2-10. The radiohumeral and ulnohumeral joints of the elbow are contained in the elbow joint capsule.


It is very important to accommodate the carrying angle when fabricating elbow extension orthoses to ensure that the joints and soft tissues are positioned correctly. A less common deviation is cubitus varus, or gunstock deformity, where the forearm deviates medially in relation to the distal humerus. Collectively, the terms valgus and varus are derived from the Latin turned outward and turned inward, respectively (Figure 2-12).



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Figure 2-11. Carrying angle: the oblique orientation of the humerus in relation to the radius and ulna with the UE in anatomical position.




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Figure 2-12. (A) Varus deformity of the elbow. (B) Valgus deformity of the elbow.




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Figure 2-13. The muscles on the anterior aspect of the arm include the brachialis, biceps brachii, and brachioradialis.


The elbow has very strong ligamentous support on the medial and lateral aspects of the joint. The medial collateral ligament complex contributes to stability of the joint and helps limit valgus stresses, whereas the lateral collateral ligament complex helps to limit varus stresses. Injury to these structures can therefore significantly affect stability and movement of the elbow.


Soft Tissue Structures


Muscles in the upper arm are divided into two compartments: anterior and posterior. The anterior compartment, shown in Figure 2-13, comprises muscles that flex the elbow (biceps brachii, brachialis, and brachioradialis) and help flex and adduct the glenohumeral joint (coracobrachialis). The musculocutaneous (biceps brachii and brachialis) and radial (brachioradialis) nerves innervate this compartment. The three heads of the triceps brachii (long, lateral, and medial), innervated by the radial nerve, make up the posterior compartment of the arm.



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Figure 2-14. The cubital fossa contains many important anatomical structures.


The volar aspect of the elbow contains a number of important soft tissue structures and is termed the cubital fossa. This fossa, or hollow, is bordered by the brachioradialis muscle laterally, pronator teres muscle medially, biceps brachii superiorly, and biceps tendon inferiorly (Figure 2-14A and B). Contents of the cubital fossa include the median nerve, brachial artery, and distal biceps tendon.


The posterior aspect of the elbow, in contrast, is bony in nature and consists of the olecranon process in the center, the lateral epicondyle laterally, and the medial epicondyle medially. When the elbow is flexed at 90 degrees, these three structures form a triangle. In extension, they form a straight line (Figure 2-15).


The area between the medial epicondyle and the olecranon is referred to as the cubital tunnel, and the ulnar nerve travels through this area on its course from the arm into the forearm (Figure 2-16). Collectively, the bony structures and the ulnar nerve can be subject to irritation and pressure from an elbow and forearm immobilization orthosis and must be protected during fabrication.



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Figure 2-15. Note the triangle formed by the olecranon, medial and lateral epicondyles with the elbow flexed at 90 degrees and the straight line formed with the elbow extended.




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Figure 2-16. The ulnar nerve travels in a space between the medial epicondyle and the olecranon. These structures must be protected in orthoses that include the elbow.


WRIST


The wrist joint is made up of several distinct joints: distal radioulnar, radiocarpal, midcarpal, and CMC joints of digits II to V (Figure 2-17).


Soft Tissue Structures


Muscles


Muscles in the forearm and hand can be divided into extrinsic and intrinsic groups. Extrinsic muscles originate proximally in the forearm and insert on or distal to the wrist. Muscles on the dorsal or posterior aspect of the forearm are commonly referred to as the extrinsic extensors (Figure 2-18); muscles on the volar or anterior aspect are extrinsic flexors (Figure 2-19). Intrinsic muscles originate and insert distal to the wrist (Figure 2-20).


Refer to Table 2-1 for a complete list of forearm and wrist muscles and their nerve innervations.


Muscles in the intrinsic group work in a synergistic manner to provide stability for effective coordination, dexterity, and power (Table 2-2).


Flexor and Extensor Retinaculum


The retinacula collectively are strong fibrous bands that serve to hold the tendons that cross the wrist close to the wrist joint axis during movement. This helps to optimize the forces that the muscles are able to generate during flexion and extension of the wrist and fingers. The flexor retinaculum courses across the volar aspect of the wrist and is continuous with the transverse carpal ligament. The eight flexor digitorum superficialis (FDS) and profundus tendons (each muscle has four tendons), the flexor pollicis longus tendon, and the median nerve run beneath the flexor retinaculum. Wrist and hand orthoses are commonly prescribed for conditions that affect these structures, such as tendonitis, carpal tunnel syndrome, and hand trauma (Figure 2-21).



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Figure 2-17. Note the joints that make up the wrist: distal radioulnar, radiocarpal, midcarpal, and CMC joints of digits II through V.


The extensor retinaculum, located on the dorsal aspect of the wrist, lies on top of the extensor tendons of the wrist and fingers (Figure 2-22). Six distinct compartments under the extensor retinaculum serve to group the tendons together according to their function (Box 2-2). Hand and wrist immobilization orthoses may be prescribed for treatment of inflammation of the tendons in these compartments.


THUMB


The human thumb is critical to hand function and contributes significantly to grip, pinch, and fine motor movements. The CMC joint, or basal joint, formed by the base of the first metacarpal and trapezium, is considered to be the most important joint of the thumb and hand. This joint possesses a wide arc of motion, largely due to the saddle shape of the joint surfaces. Combined with strong ligamentous support and intrinsic musculature, this relationship permits movements unique to this joint, circumduction and opposition, along with stability during pinching and gripping tasks.



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Figure 2-18. (A) The extrinsic extensors on the dorsal aspect of the forearm and wrist: superficial layer. (B) The extrinsic extensors: deep muscles.

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Mar 24, 2020 | Posted by in PHYSICAL MEDICINE & REHABILITATION | Comments Off on Anatomical and Biomechanical Principles of Orthotic Intervention
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