ASTM F2724 Test Method of Dislocation mobile bearing Knee

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ASTM F2724 Standard Test Method for Evaluating Mobile Bearing Knee Dislocation

ASTM F2724 provides a standardized in‑vitro method to evaluate whether a mobile‑bearing total knee can resist dislocation modes where the mobile insert either:

Spin‑out: excessive rotation of the bearing (rotating platform / multidirectional platform) leading to loss of articulation between femoral or tibial components and the mobile bearing.

Spit‑out: the bearing escapes anteriorly or posteriorly from beneath the femoral component.

Test Principle

Clinically, mobile bearings can get into trouble when the femoral condyles are off‑centered (often biased toward one condyle) and then the bearing is driven anteriorly or posteriorly while a joint compressive force is maintained. That combination can make the femoral–bearing contact walk off the rim → spin‑out (rotational loss of congruity) or spit‑out (bearing leaves the femoral “capture” zone).

ASTM F2724 therefore creates a conservative, worst‑case mechanical provocation:

Uses a two‑axis orthogonal load frame to apply a superior–inferior (S/I) compressive joint force while commanding anterior/posterior (A/P) displacement of the femur relative to the tray.

Forces an extreme single‑condyle dominance: 80% medial / 20% lateral load split (by pivoting about an axis offset from the centerline) to increase the chance of walking off the bearing rim.

Tests at several flexion angles (including max flexion) because capture behavior changes strongly with flexion geometry.

Measures how far the femur can translate before contact is lost, and records the peak loads at that event, plus which mode (spin‑out vs spit‑out) occurs first.

Key Terminology (the definitions the method relies on):

Term	Meaning in ASTM F2724
Mobile bearing (insert)	Component between fixed femoral & tibial components; articulating on both inferior and superior sides
Neutral point	Midpoint of the bearing axis
Bearing axis	Line connecting the lowest points on lateral & medial condyles of the superior surface of the mobile bearing
Centerline axis	Line through the neutral point, perpendicular to bearing axis, lying in a plane parallel to the flat portion of the inferior articulating surface at 0° posterior slope
Spin‑out	Excessive rotation of the bearing such that there is dislocation between femoral/tibial components and the mobile bearing
Spit‑out	Bearing escapes anteriorly or posteriorly from beneath the femoral component
Total bearing spacing	Distance between contact points as given by Test Method F1223

Test Specimen Information

Components: Femoral component + mobile bearing (UHMWPE insert) + tibial tray/baseplate.

Worst‑case size justification: You must perform an engineering analysis of all sizes to justify which size is the worst case; then test ≥5 inserts of that size.

Reuse: Tibial tray & femoral component may be reused for multiple trials if undamaged.

Insert sterilization: Tibial inserts shall undergo sterilization as normally employed with actual implants (because that affects UHMWPE surface/mechanical state).

Conditioning: Expose to clean atmosphere, 25 ± 5 °C, 24 h prior to testing.

Test Equipment Required for ASTM F2724:

Component	Specifications
Two‑axis dynamic loading tester	Dual‑axis force/displacement control; real‑time force and displacement recording. Recommend UnitedTest UTDST series.
Custom fixtures	Support varus/valgus tilt; apply 80% medial / 20% lateral load distribution through femoral condyles
Alignment tools	Set tibial posterior slope and femoral flexion per surgical recommendations
Lubrication	Light water coating (RO/DI/distilled) to reduce friction during testing

Specific Test Parameters

Parameter	Stipulation
Joint reaction force (S/I)	710 N (160 lbf) applied and held constant
A/P displacement rate	1 mm/s (posterior pull‑off until contact lost; then re‑align & go anterior)
Flexion angles tested	0°, 60°, 90°, and maximum flexion angle the implant is intended to achieve
Condylar load bias	80% medial / 20% lateral via pivot offset set to 30% of total bearing spacing medial from the centerline axis
Tibial slope	Set to the recommended surgical posterior slope for the device
Temperature	Room temperature (components conditioned at 25 ± 5 °C / 24 h before test)
Lubrication	Bearing surfaces lightly coated with water (RO/DI/distilled) to reduce friction effects
DOFs (constraints)	Femur: free to translate A/P under actuator control; constrained in other translations/rotations. Tibial tray: mounted to allow varus/valgus tilt about the calculated pivot.
Starting position	Zero rotation; femur seated at the approximate low point of both condylar surfaces under a 100 N preload before A/P pull‑off

Step-by-step ASTM F2722 Test Procedures

1, Select flexion angle from the set: 0°, 60°, 90°, and max flexion.

2, Mounting:

Tibial tray at recommended posterior slope; fixture provides the medial‑biased pivot (80/20).

Femoral component set to the target flexion, constrained appropriately, free in A/P.

3, Seat the femur: zero rotation; position femur on the articulating surface near the low points of both condyles under ~100 N load.

4, Apply joint load: ramp to 710 N S/I, hold constant.

5, Posterior pull‑off: displace femur posteriorly at 1 mm/s until contact is lost with either condyle (“spit‑out” tendency). Record total femoral displacement relative to tray and maximum load.

6, Reset: realign to original position, reapply 710 N, then

7, Anterior pull‑off: displace femur anteriorly until contact lost; again record displacement & peak load (and which failure mode occurs first, spin‑out or spit‑out).

8, Repeat for the remaining flexion angles per your plan.

Test Application (Industry Field)

Industry: Orthopedic implant OEMs (mobile‑bearing TKR platforms), contract/ISO 17025 biomechanical labs, and regulatory engineering files (design verification evidence).

It specifically addresses a known clinical failure mode: dislocation/subluxation of meniscal / rotating‑platform bearings (literature rates historically < ~9.3% per Appendix X1.1).

It is not for fixed bearings, and it does not replace soft‑tissue constraint—so it’s used together with judgment/other constraint data (e.g., ASTM F1223) and with recognition that patient soft‑tissue status matters.

Related Test Standard:

Designation	Title	What it purpose
ASTM F1223	Test Method for Determination of Total Knee Replacement Constraint	F2724 borrows its load level concept (710 N) and uses total bearing spacing as defined/measured per F1223.
ASTM F2083	Specification for Knee Replacement Prosthesis	Provides the method to determine / justify the maximum flexion angle at which the femoral component is mounted
ASTM F2722	Practice for Evaluating Mobile Bearing Knee Tibial Baseplate Rotational Stops	Stop-feature overload under deep-flexion rotation
ASTM F2723	Test Method for Evaluating Mobile Bearing Knee Tibial Baseplate/Bearing Resistance to Dynamic Disassociation	Bearing jumping out of the tray under dynamic conditions
ASTM F2724	Test Method for Evaluating Mobile Bearing Knee Dislocation	Dislocation events
ASTM F2777	Test Method for Evaluating Knee Bearing (Tibial Insert) Endurance & Deformation Under High Flexion	UHMWPE tibial insert posterior-edge fatigue / creep / fracture (axial, not rotational)

Why This Test Is Important for the Material (UHMWPE) & Design

Even though ASTM F2724 is quasi‑static, it still stresses how the UHMWPE bearing’s geometry, stiffness, and surface behavior interact with capture features:

Rim/edge geometry & clearances matter more than bulk strength

Spin‑out/spit‑out is usually a geometrical capture problem: rim height, wall angle, chamfer radii, superior–inferior clearance, and how the femoral condyle “rides” the periphery. UHMWPE’s modulus & creep influence whether those clearances shift over time and reduce capture margin.

Friction & surface state influence walk‑off distance

That’s why it calls for a light water coating—to keep friction in a sane range and avoid artificially high stick‑slip that could mask or exaggerate propensity to spit‑out.

It exposes marginal designs that look fine at mid‑flexion but fail at extremes

By forcing max flexion and an 80/20 single‑condyle bias, F2724 flushes out designs where the condyle can “fall off” the bearing rim or where the bearing can rotate sufficiently to break congruity.

Comparative readout is actionable

The outputs (translation‑to‑loss‑of‑contact, peak loads, first failure mode) let you rank designs/insert thicknesses/rim profiles on the same mechanical playing field, which is exactly what design verification needs before you trust it in patients.

Multi‐Axis Fatigue Torsional & Bending testing system understake this task, used to check the torsion and bending, tension test for the Intramedullary lengthening nail/Intramedullary leg lengthening implants nails.

Electronic Dynamic Testing Machine for high-precision cyclic and vibration testing of materials and components, featuring accurate frequency control, stable performance, and laboratory-grade reliability.

Q1: What does F2724 actually do?

A: It's a quasi-static dislocation screen for mobile-bearing total knees. It holds the implant under a constant compressive force (710 N / ~160 lbf), biases the load so 80% is on the medial condyle (a worst-case single-condyle scenario), and then slides the femur forward and backward until the condyles lose contact with the bearing — i.e. the bearing either spins out (rotates excessively and loses congruity) or spits out (escapes anteriorly/posteriorly from under the femur).

Q2: What's the difference between spin-out and spit-out — and which one am I really worried about?

Term	What happens	Typical root cause
Spit-out	The bearing escapes from beneath the femoral component — anteriorly or posteriorly	Excessive A/P translation, insufficient superior capture (rim height, wall profile), or condylar overhang
Spin-out	Excessive rotation of the bearing (rotating / multi-directional platform) such that there's dislocation between femoral or tibial components and the mobile bearing	Bearing rotates far enough that the femur is no longer congruently seated on the bearing surface

Q3: We already run ASTM F1223 for constraint. Why add F2724?

A: F1223 measures things like A–P draw and rotary laxity — it characterizes how much the whole TKR systemmoves under load, including condylar liftoff patterns.

F1224 isolates a different question: under an extreme off-centered load, how far can the femur translate before contact is lost entirely — i.e. the jump from "constrained but loose" to actual dislocation.

F1223 doesn't tell you the margin to catastrophic loss-of-articulation. F2724 does.

The standard explicitly warns: neither test simulates soft tissues (capsule, ligaments, PCL/MCL/LCL). A patient with good soft-tissue restraint may tolerate a lower spin-out resistance; a patient with major bone loss / destroyed ligaments needs higher resistance. So F2724 is a design screening tool, not a standalone safety certificate.

Q4: Which knee implants does this test apply to?

A: Only mobile-bearing total knee designs. It does NOT apply to fixed-bearing knee systems.

Q5: Why use a 710 N joint reaction force?

A: 710 N is taken from ASTM F1223 (knee constraint test). It is used for consistent comparative evaluation between implant designs, not as a direct physiological load.

Q6: Why use 80% medial / 20% lateral load distribution?

A: This represents a worst-case clinical loading scenario (typical gait is ~60% medial / 40% lateral) to maximize the risk of dislocation and challenge the implant’s constraint.

Q7: At which flexion angles is the test performed?

A: Testing must be done at 0°, 60°, 90°, and the maximum claimed flexion angle of the implant.

Q8: How many test samples are required?

A: At least five mobile bearing inserts of the worst-case size (justified by engineering analysis) must be tested.

Q9: What is the displacement speed during testing?

A: 1 mm/s anteriorly and posteriorly until condylar contact is lost.

Q10: Can tibial trays and femoral components be reused?

A: Yes, if they show no damage during testing. Only the mobile bearing inserts need to be new for each trial.

Q11: Does this test simulate in vivo soft tissue constraints?

A: No. The test does not replicate ligaments or soft tissues, so results must be used with clinical patient condition factors.

Q12: What data must be reported?

A:Worst-case size justification

Relative femoral displacement

Maximum load recorded

First failure mode (spin-out or spit-out).

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