The constrained tumor knee as a model case by implantcast Head of Project Management Steffen Aßmann
Beyond the standard: when tumor endoprostheses outpace testing protocols
Tumor endoprostheses make functional reconstruction after major resections possible in a way that would have been hard to imagine a few decades ago. At the same time, we still assess their mechanical safety largely with test protocols that were developed for primary arthroplasty. In daily practice, this gap is obvious: longer lever arms, compromised soft tissues, modular segment chains and loading scenarios that simply do not appear in conventional knee or hip standards.
The constrained tumor knee illustrates this particularly well. The rotating-hinge mechanism often operates close to full extension, sometimes right at a mechanical hyperextension stop. Exactly in this range, peak loads occur for which we currently don’t have a standardised test block.
Why do we not yet have an adequate standard?
First, failure patterns differ considerably depending on anatomical site and reconstruction length, highlighting the limitations of a one-size-fits-all testing approach for those divers circumstances.
Second, loading data are well documented for standard total knee arthroplasty, but much less so for ligamentously insufficient and oncologic knees – precisely the situations in which we use these implants at the edge of what they can tolerate.
Third, modularity and long lever arms introduce extra weak points: diaphyseal segments, adapters and hinge bearings.
Clinically, we have been seeing recurrent patterns in coupled knees for years in any kind of prosthesis and from any manufacturer: wear and damage of the PE insert, an increase in hinge laxity, wear and damage of the coupling in hyperextension, and load shifting into adjacent components. On top of this comes the question of what loads actually reach the stem-bone interface at different reconstruction lengths. From a mechanical perspective it seems plausible that, with increasing reconstruction length, the effective lever arm grows and bending and torsional moments rise. Whether this, under real-world conditions – with an active soft-tissue envelope, muscle forces and ligament tension – always results in higher loads at the stem, is not clearly established and needs more systematic investigation.
Towards a pragmatic testing concept
The route to more realistic testing does not require an entirely new universe of standards. What we need is a clear framework that links clinical observation, simulation and bench testing in a meaningful way. The coupled tumor knee is well suited as a model case against other reconstructions can be aligned with.
Clinic first: gait analysis highlights the critical phases
Three-dimensional gait analysis combined with ground reaction force measurements shows when in the gait cycle things become critical – typically at heel strike with the knee in full or even slight hyperextension. Gait analysis does not tell us how the load is distributed between implant and soft tissues, but it does define the time windows that our test rigs need to reproduce.
In-silico as a separate pillar: "how much?" and "where?"
Advanced musculoskeletal simulations now couple the forces from muscles and ligaments with joint contact behavior. In this way they allow us to quantify load transfer and moments at the PE insert, at junctions and at the stem.
Such specifically parameterisable musculoskeletal models can be used to systematically vary, for example:
- the degree of soft-tissue insufficiency (e.g. of the extensor mechanism),
- the reconstruction length, and
- the ratio of segment length to residual bone length.
When implemented in a patient-specific workflow, these simulations can also support implant selection and positioning and help identify where current designs may be optimized. From the resulting scenarios, worst-case profiles for moments, contact forces and joint angles can be derived. These can be translated directly into boundary conditions for mechanical testing providing a basis for subsequent FEM analyses that drive further refinement of the implants.
Reversed biomechanical testing: simulation meets hardware
The next step is to reproduce these profiles on the test bench. In a simulator, the hyperextension pattern derived from clinical data and in-silico analysis is intentionally applied, and the forces and moments required to generate it are recorded.
For the constrained knee, this allows us to define clear acceptance criteria for all kinds of mechanical aspects, such as:
- wear volume and loss of geometry of the PE insert,
- increase in hinge laxity or wear of the coupling, and
- the absence of unintended metal-to-metal contact within the hinge.
This approach clarifies the relevant mechanical load scenarios and can be used to identify targeted design optimizations in current designs.
Using existing standards - but translating their content
In addition to existing test standards for primary knee prostheses, which can be adapted and applied to some extent, standards developed for exoprostheses (such as ISO 10328) may also be relevant. However, this requires a detailed understanding of this specific type of prosthetic system and its underlying principles.
While these standards cannot be adopted one-to-one, they offer ready-made testing logics that can be adapted to tumor scenarios, provided that lever arms, embedding conditions, and angles are clearly defined in a tumor-specific manner.
Testing the whole construct - not just individual parts
Tumor endoprostheses rarely fail at an isolated component. Failures usually emerge from the interaction of several modules. Test setups should therefore address the entire implant construct – for example, using four-point bending with superimposed torsion across the full reconstruction, including all modular segments and adapters.
In such a configuration, clinically relevant endpoints include crack initiation or fracture, excessive play, and loss of function of the reconstruction – precisely the kinds of complications we observe in real-world cases.
Explants as the reality check
Ultimately, real failure patterns are essential for understanding the failure mechanism. Analyses of retrieved implants – for example, loss of geometry at the PE insert, wear tracks in the hinge, tribocorrosive damage at junctions or abnormalities at the stem-bone interface – should feed directly back into the design of test profiles and acceptance criteria.
This allows us to identify specific loading conditions that the implant can no longer withstand over time. The magnitude of the forces and moments required to induce failure can then be compared and interpreted in light of the insights gained from previews points.
In summary, this does not result in a completely new “standard cosmos”, but in a modular test program that uses existing standards in a meaningful way and, for the first time, systematically reflects oncologic loading scenarios. For manufacturers, it offers clearer benchmarks and development targets; for clinicians, it improves comparability; and for regulators, it provides a transparent pathway – in particular with regard to specific federal regulatory requirements and the longer-term goal of ISO or ASTM establishment.
Where do we go from here?
The coupled tumor knee is more than an exotic niche. It concentrates many of the mechanical challenges we face in limb salvage surgery. If we manage to define a robust test concept here, we will have a blueprint that can conveyed to other complex reconstructions.
A pragmatic next step would be to agree on a pilot test profile specifically for rotating-hinge tumor knees under clinically relevant conditions. Such a profile should combine:
- a clearly defined input derived from gait analysis and in-silico work,
- a set of measurable mechanical endpoints (PE wear and geometry loss, hinge laxity, metal contact), and
- a transparent mapping to existing ISO/ASTM test logics.
On this basis, individual centers and laboratories can begin to map their current testing routines against the proposed profile, identify gaps and, where appropriate, incorporate additional elements. Even without a formal standard in place, this would already move us closer to comparable, clinically grounded test conditions for coupled tumor knees.
In the longer term, this shared experience could lead into a consensus “recommended practice” for rotating-hinge tumor knee testing. Such a document would not replace formal ISO or ASTM work, but it would provide a mature starting point for any future standardization effort. In addition, it would make our current design decisions and mechanical safety arguments more traceable and easier to explain to regulators as well as to clinicians and biomechanists.