Introduction to Kinematics

For Medical Students

The Principle

Kinematics is the description of motion: where something is, how far it has moved, how fast, and how that speed is changing. It leaves out the forces entirely.

Kinetics is the other half, and it deals with the forces and moments that produce the motion. 3.4 covers it.

Keeping the two apart is useful rather than pedantic. Two knees can move identically while one is doing far more work, and only the kinetic description separates them.

At the Knee

The quantities are position, displacement, velocity and acceleration, each of which can be angular or linear. A knee has both, because it rotates and it translates.

Angular kinematics covers flexion and extension, axial rotation, and varus and valgus angulation. Linear kinematics covers the translations, which are small and matter most to ligaments.

A kinematic description needs a reference: which bone is held still, and which axes the motion is measured against. The same movement described from the femur and from the tibia gives two sets of numbers.

3.2.1 names the six degrees of freedom this chapter then describes through range.

What Changes It

The activity. A knee flexing in a seated leg extension and the same knee flexing in a squat follow different paths, which is why 3.3.3 and 3.3.4 are separate pages.

Speed. Faster motion changes the muscle activity that guides it, and therefore the path, even when the range is the same.

Load. A loaded joint is compressed, and a compressed joint follows a tighter path.

The individual. Joint shape varies, so a normal range is a population statement rather than a prediction for one person.

Clinical Relevance

A kinematic description is what a clinician actually observes. Watching a patient stand up from a chair is kinematic observation.

Describing the motion first and the cause second avoids a common error, which is naming a muscle as the problem before establishing what the motion is.

Every activity page in this chapter gives the pattern a normal knee follows, which is the comparator a departure is read against. Gait pathology belongs to 3.14.

Numbers quoted for a range of motion come from a method. Knowing the method is part of knowing the number.

MRCS Level

The Principle

Define the frame before the figure. Knee kinematics is reported in a joint coordinate system, usually the one credited to Grood and Suntay, with a femoral flexion axis, a tibial long axis and a floating axis perpendicular to both.

Rotations and translations are then reported about and along those three axes, which is how two laboratories can compare results.

Order matters in three dimensions. The same three rotations applied in a different sequence put the bone in a different place, which is why the sequence is part of the convention rather than an afterthought.

At the Knee

Flexion and extension is the largest rotation and the one every other quantity is reported against, so a kinematic figure without a flexion angle is incomplete.

Axial rotation and the translations are an order of magnitude smaller, and they are where measurement error competes with the quantity being measured.

Contact point position is a kinematic quantity too, and it migrates posteriorly on both condyles with flexion. 3.5 covers contact mechanics.

Open chain and closed chain motion describe the same relative movement with a different segment fixed, and the clinical tasks divide along that line.

What Changes It

Weight bearing, which changes the path enough that non weight bearing laboratory data cannot be applied to a standing patient without saying so.

Muscle activity, which guides the path within the limits the passive structures allow.

Soft tissue artefact, which is a measurement effect rather than a real change, and is largest for the small rotations.

Structural injury or previous surgery, both of which widen the path rather than add a motion of a new kind. 3.2.9 covers that.

How It Is Measured

Optical motion capture with skin markers is the standard laboratory method, and its limit is soft tissue artefact between the marker and the bone.

Fluoroscopy, single or biplanar, measures bone directly and removes that artefact. Its limits are the field of view and the radiation dose.

Cadaveric rigs and robotic testing measure all six degrees of freedom under a known load, in a specimen with no muscle activity.

Goniometry and inertial sensors measure a single plane adequately and the smaller rotations poorly, which is the trade for using them outside a laboratory.

Clinical Relevance

State the method and the loading condition with any kinematic figure, because the same knee gives different numbers under different methods.

Treat a non weight bearing measurement as describing a different task from the weight bearing one, not a more convenient version of it.

Read a small rotational difference with the measurement error in mind before calling it a finding.

Use the activity pages in this chapter as the normal comparator rather than a single quoted range of motion.

FRCS Level

At the Knee

Treat kinematics as the language in which a reconstruction's aim is stated. Restoring stability is a kinetic claim about restraint, and restoring motion is a kinematic claim about path, and an operation can achieve one without the other.

Abnormal kinematics with normal laxity is a recognised pattern after reconstruction. The arthrometer reports the limit and the patient experiences the path.

Arthroplasty is a kinematic problem explicitly, since an implant imposes a path through its geometry rather than negotiating one with the soft tissues. 3.16 covers the advanced material.

What Changes It

Implant or graft geometry, which replaces part of the constraint set and therefore the path.

Alignment, which changes the loading that holds the joint on its path.

Soft tissue balance, which decides whether the imposed path and the preferred one agree.

The task being measured, which is why a kinematic claim about an operation is only as general as the activities it was tested in.

How It Is Measured

Biplanar fluoroscopy during a functional task is the reference method in vivo, and its activity range is limited by the equipment rather than by the question.

Model based tracking against a computed tomography or magnetic resonance model improves accuracy and adds a registration step that carries its own error.

Robotic cadaveric testing answers apportionment questions that cannot be asked in a living knee, and it answers them without muscle.

Patient reported outcome is not a kinematic measure and is the endpoint the kinematic ones are proxies for, which is worth stating when the two disagree.

Clinical Relevance

Separate the kinematic and the kinetic claim when appraising a construct, since a paper usually measures one and discusses the other.

Expect restored laxity without restored motion, and ask about function rather than inferring it from an instrument.

Judge a kinematic study by the task it measured before generalising it to a patient's activity.

Record the loading condition in any comparison between operated and unoperated sides.