StemFit — Personalized THA (powered by CPAH)

1 · Calibration not calibrated

Place a radio-opaque marker of known size on the film. Calibration is required for offset in mm; NSA, cortical index and FOR are ratios and remain valid without it.

Marker type Diameter (mm)

2 · Measurement steps

Measurements

Enter values manually / add lateral CTI

NSA (°) Cortical Index FOR Lateral CTI (opt)

CPAH classification

Complete the shaft, head, neck and cortical measurements to classify.

Pick a site (＋ Site for multiple hospitals), tick Stock for the stems it carries, then Save. Recommendations can then be filtered to the active site's formulary.

Stock	Stem	Manufacturer	Geometry	Length	Modularity	Collar	Surface	Use	Code	Src

The CPAH classification

The Coronal Plane Alignment of the Hip (CPAH) classification defines nine femoral morphotypes by combining an intramedullary parameter (Dorr canal shape) with an extramedullary one (neck–shaft angle), each further split by the femoral offset ratio.

Fig. — CPAH matrix: Dorr A–C × coxa vara / norma / valga, with normal (FOR_N) and high-offset (FOR_H) subtypes. © CPAH study group.

How each parameter is measured

Radiological measurement technique — Fig. — Radiological measurements on a calibrated low-centred AP pelvis. © CPAH study group.

Cortical index (Dorr): CI = (Z − X) / Z, measured 10 cm distal to the mid-lesser-trochanter line, where Z = femoral diaphyseal (outer) diameter and X = intramedullary canal diameter. Dorr A > 0.60 · B 0.50–0.60 · C < 0.50.
Neck–shaft angle (NSA): angle between the femoral neck axis and the femoral shaft axis. Coxa vara < 120° · norma 120–140° · valga > 140°.
Femoral offset (FO): perpendicular distance from the femoral head centre of rotation to the shaft axis (needs calibration for absolute mm).
Femoral offset ratio: FOR = FO / Z — scale-independent. Normal < 1.60 · high ≥ 1.60.

Image quality matters: use a low-centred AP pelvis with 15° internal rotation and a neutral pelvis, and always include a calibration marker — plain radiographs magnify/underestimate offset by ~3–5 mm without one.

Bone-quality / osteoporosis screen

The cortical thickness index doubles as an osteoporosis screen. Sah et al. found CTI correlates with DXA T-score: CTI (AP) ≤ 0.50 (specificity 0.84) or CTI (lateral) ≤ 0.40 (sensitivity 0.85, specificity 0.79, LR+ 4.0) flags likely osteoporosis and should prompt a DXA referral. Dorr C also tracks lower T-scores; the canal-to-calcar ratio did not correlate with BMD. Enter a lateral CTI under “Enter values manually” for the stronger threshold.

Cemented vs cementless

The tool suggests a fixation mode from canal shape and bone quality: Dorr A / adequate cortical thickness favours cementless press-fit; a low cortical thickness index (osteoporotic bone, Sah et al.) or a Dorr C ‘stovepipe’ canal leans toward cemented fixation given the higher periprosthetic-fracture and loosening risk of cementless stems in poor bone. This is decision support only — age, activity, comorbidity and surgeon experience remain decisive.

Stem geometry (Radaelli)

A flat taper · B1/B2/B3 rectangular / quadrangular / short-quadrangular tapers · C1/C2/C3 fit-and-fill / anatomic / short fit-and-fill · D conical · E cylindrical · F calcar-guided ultra-short — with length, modularity (I–IV), collar and surface modifiers.

References

Central reference

Stauss R, Savov P, Biestmann F, Brueggemann M, Mont MA, Seyler TM, Ettinger M. Definition of Femoral Morphotypes Based on the Coronal Plane Alignment of the Hip Classification. The Journal of Arthroplasty 2026 (in press). — the CPAH classification this app implements.

Femoral stem classification & fixation

Radaelli M, Buchalter DB, Mont MA, Schwarzkopf R, Hepinstall MS. A New Classification System for Cementless Femoral Stems in Total Hip Arthroplasty. J Arthroplasty 2023;38:502–510.
Khanuja HS, Vakil JJ, Goddard MS, Mont MA. Cementless Femoral Fixation in Total Hip Arthroplasty. J Bone Joint Surg Am 2011;93:500–509.
Kheir MM, Drayer NJ, Chen AF. An Update on Cementless Femoral Fixation in Total Hip Arthroplasty. J Bone Joint Surg Am 2020;102:1646–1661.
Siddiqi A, Dayton S, Yousuf KM, Chen AF. Cementless Femoral Stems in Primary Total Hip Arthroplasty. Review article.
Cemented Stem Design in Total Hip Arthroplasty: Fixation Philosophies, Biomechanics, and an Updated Classification System. J Am Acad Orthop Surg 2024;32:525–534.

Femoral morphotype & anteversion

Sariali E, Boukhelifa S. Unlike Acetabular Anteversion, Femoral Anteversion Is Not Associated with the Hip Coronal Morphotype: An Anatomic Basis for a New Hip Morphotype Classification at Total Hip Arthroplasty. J Bone Joint Surg Am.

Bone quality & the Dorr classification

Dorr LD, Faugere MC, Mackel AM, Gruen TA, Bognar B, Malluche HH. Structural and Cellular Assessment of Bone Quality of Proximal Femur. Bone 1993;14:231–242.
Wilkerson J, Fernando ND. Classifications in Brief: The Dorr Classification of Femoral Bone. Clin Orthop Relat Res 2020;478:1939–1944.
Sah AP, Thornhill TS, LeBoff MS, Glowacki J. Correlation of Plain Radiographic Indices of the Hip with Quantitative Bone Mineral Density. Osteoporos Int.

Femoral offset

Lecerf G, Fessy MH, Philippot R, Massin P, Giraud F, Flecher X, Girard J, Mertl P, Marchetti E, Stindel E. Femoral Offset: Anatomical Concept, Definition, Assessment, Implications for Preoperative Templating and Hip Arthroplasty. Orthop Traumatol Surg Res 2009;95:210–219.
Dastane M, Dorr LD, Tarwala R, Wan Z. Hip Offset in Total Hip Arthroplasty: Quantitative Measurement with Navigation. Clin Orthop Relat Res 2011;469:429–436.

Terms & medical disclaimer

⚠️ Not a medical device. StemFit (powered by the CPAH classification) is an educational and research decision-support tool. It is not FDA/CE cleared or registered and is not intended to diagnose, treat, or make or confirm any clinical or surgical decision. All measurements, classifications, bone-quality flags, fixation guidance and stem recommendations are approximations dependent on radiograph quality, positioning and calibration, and must be independently verified by the operating surgeon, who remains solely responsible for all clinical decisions. The software and its data (including inferred/possibly outdated stem classifications) are provided “AS IS” without warranty of any kind; verify stem specifications against the manufacturer’s current instructions for use. To the maximum extent permitted by law, the authors and developers accept no liability for any damages arising from use of or reliance on this tool. Do not upload patient-identifying data; you are responsible for de-identification and for compliance with HIPAA, GDPR and your institution’s policies. Figures © the CPAH study group, used with permission. Use constitutes acceptance of these terms.

Study dashboard

Registered users

Name	Email	Institution	Approach	Fellowship	Age	Gender	Location	Paid	Collab?	Joined	Reg IP

Submitted cases & radiographs

Saved classifications with their recommendations. Radiographs are stored only when the uploader consented to AI-training use. IP address, timezone and timestamp are recorded with each submission.

When (UTC)	User	CPAH	Top recommendations	Radiograph	IP	TZ

Personalized total hip arthroplasty — from the femur up.

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