# Example: Hip Implant Material Selection (Biomedical Engineering)

**Status:** Draft
**Phase:** The Bedrock Phase

## What This Example Demonstrates

Context record structure (OE-0003) in biomedical engineering, including cross-discipline constraints and regulatory considerations.

## The Observation

A patient cohort (n=340) receiving cobalt-chrome (CoCr) hip implant bearings showed serum cobalt ion elevation above 1.0 ppb in 8.2% of patients at 5-year follow-up, with 2.1% requiring revision surgery for adverse local tissue reaction (ALTR).

## Engineering Translation

Material selection in biomedical engineering involves constraints from multiple domains: materials science, biology, regulatory requirements, and patient outcomes. The reasoning behind a material choice must be preserved because a future engineer may need to evaluate whether the material is still appropriate as new evidence emerges or as the patient population changes.

## Context Record

| Field | Content |
|---|---|
| **Decision** | Select ceramic-on-ceramic (CoC) bearing surfaces for total hip arthroplasty in patients under 65 with expected activity level above the 75th percentile for age group |
| **Observation** | (1) Serum metal ion data from CoCr revision study (internal, 2023, n=340). (2) Published registry data: Australian NJRR 2022 report shows 15-year implant survival of 94.2% for CoC vs. 91.7% for CoCr in patients under 65. (3) Corroborated observation: multiple independent biomechanical studies confirm that ceramic wear debris generates a less severe inflammatory response than metallic wear debris. |
| **Alternatives** | (A) CoCr-on-CoCr — rejected for high-activity patients due to metal ion elevation risk (Observation 1). (B) Ceramic-on-polyethylene (CoP) — rejected for patients under 65 with high activity due to polyethylene wear rates at 15+ year horizon. (C) Oxinium-on-polyethylene — considered but insufficient long-term registry data for patients under 65 at current follow-up horizon. |
| **Constraints** | FDA 510(k) clearance required for bearing combination. Must demonstrate 15-year survivorship probability above 90% per registry criteria. Must not generate wear debris exceeding osteolytic threshold. Patient age and activity level restrict material options differently than geriatric population. |
| **Reasoning** | CoC provides the lowest wear rate of available bearing options (approximately 0.001 mm/year vs. 0.05 mm/year for CoP). For patients under 65 with high activity, the cumulative wear volume over a 20+ year implant life makes wear rate the dominant failure mode. The ceramic fracture risk (approximately 0.02% at 10 years with modern delta-ceramic) is acceptable given the patient population's life expectancy. The trade-off is acceptable: higher ceramic fracture risk at implantation, lower long-term wear-related revision risk. |
| **Verification** | (1) Finite element analysis of contact stress distribution confirmed peak Hertzian stress below ceramic fracture toughness threshold. (2) Simulator testing (ISO 14242-1) at 10 million cycles showed volumetric wear of 0.8 mm3 (below osteolytic threshold of 50 mm3). (3) Registry correlation: 5-year clinical data from three independent registries (AU, UK, NZ) confirm survival probability consistent with lab predictions. |
| **Lineage** | Builds on material selection framework established in CR-BIO-2019-003 (geriatric population bearing selection). Extends that framework to high-activity younger patients using updated registry data. |
| **Assumptions** | Ceramic fracture toughness will not degrade below threshold in vivo over 20+ year horizon. Patient activity level remains above 75th percentile. No novel ceramic manufacturing defects at scale. Regulatory pathway for CoC in this specific patient subpopulation remains available. |
| **Open Questions** | At what activity level does CoC's advantage over CoP become statistically insignificant? Should patients with known ceramic sensitivity be excluded from CoC consideration? |

## Self-Fading Assessment

This example transports the reader from the abstract context record structure to a concrete biomedical engineering decision involving multi-domain constraints. Once the reader understands how the same eight-field structure captures reasoning in a fundamentally different engineering discipline, the example has served its purpose.