Arthrokinematics and Joint Motion: Comprehensive Notes
Arthrokinematics: Key Concepts
Arthrokinematics (joint motion) is the movement of one joint surface relative to another.
Osteokinematics refers to physiological (voluntary) motion such as flexion/extension and is measured as range of motion (ROM) in degrees.
The focus here is on accessory joint motions: roll, slide (glide), and spin, which enable normal physiological motion.
These accessory motions occur at the joint surface and are essential even though they are smaller than osteokinematic movements.
A clear understanding of arthrokinematics is necessary for accurate palpation, ROM assessment, and manual therapy techniques.
Osteokinematics vs Arthrokinematics
Osteokinematics: physiological motion of bones around an axis (e.g., flexion/extension).
Arthrokinematics: motion of joint surfaces relative to each other (roll, slide, spin).
Note: Arthrokinematics underpins the ability to achieve, reproduce, and restore proper osteokinematic ROM.
Spin
Spin is pure rotation of the movable joint surface on the fixed adjacent surface.
Many points on the convex surface contact the same point on the concave surface during spin.
Spin rarely occurs in isolation; it typically occurs with rolling and sliding in combination.
Practical takeaway: spin contributes to the rotational component of joint motion but is rarely the sole motion.
Slide (Glide)
Slide is pure translation; linear movement of a joint surface parallel to the plane of the joint surface.
The same point on the convex surface contacts multiple points on the concave surface during slide.
Direction of sliding depends on whether the moving surface is concave or convex (Convex–Concave Rule).
Glide is the manual-therapy equivalent of the slide, i.e., practitioners perform glides to facilitate joint motion.
Glide (continued)
Manual therapy synonyms: Glide = Slide (therapeutic mobilization technique).
Example terminology: "Glide" is the clinician-driven version of the natural arthrokinematic slide.
Roll
Y Roll is a combination of slide and spin.
Many points on the convex surface contact many points on the concave surface during roll.
Roll is always in the same direction as the swinging bone motion.
Implication: roll alone is rare; it usually occurs with concurrent sliding to maintain joint congruency and smooth motion.
Combining Translation and Rotation (Roll & Slide)
Many synovial joints exhibit a combination of rotation and translation during normal motion.
Example: a translation component during a squat demonstrates combined roll and slide at multiple joints.
Clinically, these small accessory motions are essential to normal joint mechanics.
Inadequate slide can impede restoration of normal motion; excessive slide can damage surrounding soft tissues.
Instant Center of Rotation (ICR) and Curvilinear Path
Joints that rotate with translation have an axis of rotation that moves in space (not a fixed axis).
The motion of a joint undergoing rotation plus slide is described by its Instant Center of Rotation (ICR), the theoretical axis of rotation for the joint at a given position.
A joint with a fixed axis has a constant ICR; joints with rotation + slide (e.g., the knee) have multiple ICRs depending on position.
Practical takeaway: the ICR provides a way to conceptualize how the joint axis shifts during motion.
Clinical Use of ICR and Fixed Axis Assumptions
In clinical evaluation, a fixed axis is used as an approximation of the actual axis of rotation to simplify measurements.
The convex side of the joint is typically used as a reference when applying this assumption for ROM measurements, prosthesis fitting, etc.
This simplification aids in practical assessment and treatment planning.
Goniometry and ROM Assessment
ROM assessment is commonly performed using goniometry to identify joint impairment.
A single-axis goniometer determines angular position of one limb segment relative to another when aligned with the joint axis.
Bottom line: understanding joint mechanics helps the clinician align the goniometer correctly to obtain accurate ROM measurements.
Convex–Concave Rule (Direction of Intra-articular Slide)
Most joints have one concave end and one convex end.
If the moving surface is convex on a fixed concave surface, the slide occurs in the opposite direction to the angular movement (i.e., roll direction is opposite to slide direction).
If the moving surface is concave on a fixed convex surface, the slide occurs in the same direction as the angular movement (i.e., slide direction matches roll direction).
Examples
Glenohumeral Joint (Shoulder)
The head of the humerus is convex; the glenoid fossa is concave.
During shoulder abduction, the convex humeral head rolls in the superior direction and slides inferiorly on the concave glenoid.
Clinically, abduction involves inferior glide to accommodate the superior roll.
Conclusion: convex-on-concave with abduction leads to inferior slide.
Metacarpophalangeal Joint (MCP)
Proximal phalanx end is concave; distal metacarpal base is convex.
During finger extension (from flexion), the proximal phalanx moves in the same direction as the phalanx itself on the convex metacarpal surface.
Conclusion: concave-on-convex with extension leads to same-direction slide relative to roll.
Knee example (femur on tibia in flexion/extension)
In extension of the femur, anterior rolling of the femoral condyles creates tension in the PCL, resulting in a posterior translational force on the femur.
In flexion, posterior rolling of the femoral condyles creates tension in the ACL, resulting in an anterior translational force on the femur.
Summary: Roll direction influences soft-tissue tension and resulting translational forces.
Open vs Closed Chain Concepts
Open chain: distal segment moves while the proximal segment is relatively fixed (e.g., standing with foot on the floor – fixed).
Closed chain: distal segment moves with the proximal segment fixed or contacting a surface (e.g., standing with foot on floor) or when the body segment is in contact with the ground.
In closed-chain movements, the knee/ankle joints may experience different arthrokinematics than in open-chain movements.
The slide and roll directions can differ between open and closed chain conditions.
Convex–Concave Rule: Clinical Mobilization (Manual Therapy)
Clinicians mobilize joints to remove restrictions that impede motion by promoting appropriate arthrokinematic slides.
Osteokinematic ROM can be restricted if arthrokinematics are restricted; mobilizations focus on restoring the necessary joint glides.
Practical implication: the direction of mobilizing forces is chosen based on the convex–concave rule to promote the desired slide and roll at the target joint.
Example: to improve shoulder abduction (where convex humeral head slides inferiorly during abduction), an inferior glide may be applied to restore improved ROM.
Clinical Connection: Joint Mobilization and ROM Restoration
Joint mobilization aims to restore the required joint slides for normal ROM.
The specific mobilizations depend on the joint’s normal arthrokinematics, including the direction of slide and roll.
For the knee, achieving normal tibial anterior/posterior slides on the femur can help restore flexion/extension ROM.
The clinical bottom line: a thorough understanding of joint mechanics is essential for effective manual therapy.
Joint Mobility, Soft-Tissue, and Stretching Considerations
Improper arthrokinematics can restrict osteokinematics and predispose to injury.
Manual therapy is a specialized skill to address these arthrokinematic restrictions.
Stretching can improve muscle–tendon complex extensibility but may not target joint tissues that limit arthrokinematic motion. Hence, stretching alone may not always increase joint ROM.
Applied Research: Wrist ROM post-injury treated with Joint Mobilizations
Study: Six patients with wrist injury received treatment combining deep heat and joint mobilizations.
Time since injury: from 6 weeks to 2.5 years prior to treatment.
ROM deficits were up to $45^{\circ}$ in flexion and extension.
Outcome after six treatments: all patients improved to within normal ROM limits and maintained $93\%$ of that motion at 1 month post-treatment.
Rationale: joint mobilizations emphasized the gliding component of wrist motion over stretching.
Source: Ultrasound and joint mobilizations for achieving normal wrist range of motion after injury or surgery: a case series. J Athl Train 2010;45(5):486–491.
Closed-pack vs Loose-pack Positions
Closed-pack position: joint surfaces are maximally congruent; ligaments and capsules are taut; joint play is minimal.
Loose-pack position: joint surfaces are not maximally congruent; ligaments and capsules are lax; joint capsule has maximum volume and joint play is available.
Directional Diagrams and Reference Landmarks (Directional Orientation)
Directional terms used in joint diagrams: anterior, posterior, medial, lateral, superior, inferior, with reference to the midline.
These directional cues help identify glide directions and joint surface movements during assessment and mobilization.
In-class Application of the Convex–Concave Rule
Students practice determining mobilization direction based on convex–concave relationships.
Example exercise: selecting glide direction for a given joint when the moving surface is convex on fixed concave, or concave on fixed convex.
Practical takeaway: use the rule to predict the necessary intra-articular slide direction to restore specific joint motion.
Quick Reference: Summary of Key Points
Arthrokinematics = joint-surface motion (roll, slide, spin); essential for normal joint function.
Spin: pure rotation; often occurs with roll and slide.
Slide (glide): translation; direction depends on concavity; same/different direction relative to roll according to convex–concave rule.
Roll: combination of slide and spin; direction aligns with bone movement.
ICR: moving-axis concept for joints with rotation + translation; fixed ICR for some joints; multiple ICRs possible.
Goniometry: ROM measurement relies on proper alignment with joint mechanics.
Convex–Concave Rule: convex-on-concave leads to opposite slide vs roll; concave-on-convex leads to same slide as roll.
Manual therapy uses joint mobilizations based on these rules to restore proper arthrokinematics and ROM.
Clinical relevance: poor arthrokinematics can limit ROM and predispose to injury; stretching alone may not fix joint restrictions.
Evidence note: wrist ROM improvement after six joint-mobilization treatments showed substantial gains and maintenance at 1 month.
Pack positions (closed vs loose) guide assessment and treatment planning.
Note: The content above mirrors the lecture transcript across topics such as spin, slide, roll, ICR, convex–concave rule, ROM assessment, manual therapy, open/closed chain considerations, clinical examples (glenohumeral, MCP, knee), and the wrist case study with its quantitative outcomes. It emphasizes the practical application of arthrokinematics to assess and treat joints while highlighting the importance of proper technique to avoid joint injury.