Joint Biomechanics
A precise understanding of joint mobility and stability requires a foundation in the biomechanical principles governing how joints move. This article defines the fundamental concepts of joint range of motion, differentiates between physiological and accessory movements, and explains the interplay between bone movement and joint surface mechanics.
Definition of Joint Range of Motion
From a biomechanical standpoint, joint range of motion (ROM) signifies the extent of movement possible at an articulation. It encompasses both the distance and direction (e.g., flexion, extension, abduction, adduction, internal rotation, external rotation) through which a joint can typically travel. ROM is a quantifiable measure, usually expressed in angular units (degrees) using goniometry or linear units (distance) for specific motions. It serves as a fundamental parameter for assessing joint function, with deviations from established norms potentially indicating pathology. Normal mobility is considered necessary for efficient human movement.
Physiological ROM: Osteokinematics
Physiological ROM refers to the macroscopic, angular movements of bones relative to each other, occurring within the cardinal planes of motion around specific joint axes. These are the classical, voluntary movements such as flexion, extension, abduction, adduction, and rotation that are readily observable and measurable. This gross movement of bones is termed osteokinematics. Osteokinematic motion can be further categorized based on the source of the movement force:
Active Range of Motion (AROM): This is the arc of motion achieved through unassisted, voluntary contraction of the muscles acting across the joint. AROM reflects not only the available joint range but also the patient's willingness to move, neuromuscular control, and muscle strength. It represents the functional range the individual can actively utilize.
Passive Range of Motion (PROM): This is the arc of motion attained when the joint is moved solely by an external force (e.g., an examiner, gravity, or equipment) without any active muscle contribution from the patient. PROM typically exceeds AROM slightly. This difference arises because passive movement eliminates limitations imposed by muscle weakness or activation deficits, and the external force can apply a gentle stretch to the elastic components of periarticular tissues (ligaments, capsule, passive muscle elements) at the end range. Furthermore, PROM avoids the potential limitation caused by the bulk of contracting muscles (soft tissue approximation). PROM assessment isolates the mechanical constraints imposed by the joint surfaces, capsule, ligaments, and passive muscle-tendon unit tension, providing a measure of the total available anatomical range.
The distinction between AROM and PROM is clinically significant. When PROM is normal or near-normal, but AROM is markedly limited, the underlying cause is less likely to be a primary joint restriction (such as capsular tightness or bony block) and more likely related to factors impairing active movement, such as muscle weakness, pain inhibition during contraction, nerve injury, or disruption of the muscle-tendon unit. Understanding these physiological motions provides the baseline for defining hypomobility as a reduction in this range and hypermobility as an increase, particularly in PROM.
Accessory Movements: Arthrokinematics and Joint Play
While osteokinematics describes the gross movement of bones, arthrokinematics focuses on the subtle, intricate movements occurring between the articulating joint surfaces themselves. These small-amplitude motions are essential for achieving full, smooth, and pain-free osteokinematic ROM. The primary arthrokinematic movements are:
Roll: A rotary movement where new points on one joint surface contact new points on the opposing surface, analogous to a tire rolling on pavement. Rolling contributes significantly to the angular displacement of the bone.
Glide (or Slide): A translatory movement where a single point on one surface contacts multiple points on the opposing surface, akin to a tire skidding or an ice skate blade moving over ice. Gliding allows the joint surfaces to remain congruent during movement.
Spin: A rotary movement where one joint surface rotates on the opposing surface around a stationary mechanical axis, like a spinning top. Examples include shoulder internal/external rotation at 90 degrees abduction or radioulnar pronation/supination.
In addition to these fundamental motions, two related concepts are important:
Joint Play: These are small, passive translatory (glide) or distractive movements available at a synovial joint that are not under voluntary control. They can only be produced by an external force. Assessing the amount and quality of joint play provides direct information about the freedom of movement at the articular surfaces and the integrity of the joint capsule and ligaments. Sufficient joint play is necessary for normal arthrokinematics.
Component Movement: These are involuntary joint motions that obligatorily accompany active osteokinematic movements but occur outside the specific joint where the primary motion takes place. A classic example is the upward rotation of the scapula that must accompany full shoulder abduction (scapulohumeral rhythm).
Arthrokinematic motions are biomechanically critical because osteokinematic movements are achieved through specific combinations of roll, glide, and spin. If these accessory movements are restricted (e.g., due to capsular tightness limiting glide), the overall physiological ROM (osteokinematics) will also be limited. Conversely, excessive or poorly controlled arthrokinematic motion, particularly excessive translation (glide), is the hallmark of joint instability.
The Interplay: Concave-Convex Rule
The relationship between the shape of the articulating surfaces and the direction of arthrokinematic glide relative to the osteokinematic movement is described by the concave-convex rule. This principle is fundamental for understanding normal joint mechanics and for guiding therapeutic interventions aimed at restoring motion.
Rule Statement:
- When a concave joint surface moves on a fixed convex surface, the arthrokinematic glide occurs in the same direction as the angular movement of the bone shaft. For example, during open-chain knee flexion (tibia moving on femur), the concave tibial plateau rolls and glides posteriorly on the convex femoral condyles.
- When a convex joint surface moves on a fixed concave surface, the arthrokinematic glide occurs in the opposite direction to the angular movement of the bone shaft. For example, during shoulder abduction (humerus moving on glenoid), the convex humeral head rolls superiorly but must glide inferiorly on the concave glenoid fossa to prevent impingement against the acromion.
Significance: Normal joint motion requires a precise combination of rolling and gliding to maintain joint surface contact and congruency throughout the ROM. If only rolling occurred, the moving bone would either impinge on adjacent structures or dislocate. The concave-convex rule allows clinicians to predict the direction of glide needed for a specific osteokinematic motion. If that glide is restricted, the corresponding osteokinematic motion will be limited. Therefore, manual therapy techniques often focus on restoring the appropriate glide, using the rule to determine the correct direction of mobilization force.
The interdependence of roll and glide is crucial. Osteokinematic motion necessitates both components occurring simultaneously to maintain joint integrity. Should the glide component be restricted, for instance by a tight posterior capsule limiting posterior glide of the humeral head, the superior roll during shoulder flexion or abduction will quickly lead to impingement of the humeral head against the acromion, halting further osteokinematic movement even if the capacity for rolling itself is not impaired. This underscores why assessing and restoring accessory glide, guided by the concave-convex rule, is often paramount in addressing limitations in physiological ROM.
Furthermore, disturbances in arthrokinematics may precede the development of noticeable limitations in gross osteokinematic ROM. Subtle restrictions in glide or spin might initially manifest as altered joint loading mechanics, increased stress on specific capsuloligamentous structures, or minor compensatory movements elsewhere in the kinetic chain, rather than an immediate, measurable decrease in the total degrees of motion. Over time, these altered biomechanics could contribute to pain syndromes, adaptive tissue changes (e.g., capsular thickening, degenerative changes ), or eventually progress to a clinically apparent loss of osteokinematic ROM. This suggests that identifying and addressing subtle arthrokinematic restrictions early may have preventative value.
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