1 Introduction

Femoral stem implant design for total hip arthroplasty (THA) has evolved continuously since its inception in the 1960s, undergoing shape, size, material, modularity, and technique adaptations.¹ Conventional uncemented femoral stem implants have longer designs (i.e., >150 mm) that bypass the femoral metaphysis for stress transfer.^2,3 The association of longer stem designs with higher technical complexity, increased incidence of thigh pain, stress shielding, periprosthetic fractures, and increased revision surgery intricacy, along with the advent of less-invasive surgical approaches, sparked the design of shorter stem implants.^4–9

Short stem implants have lengths less than 120 mm^10,11 and primarily engage the metaphyseal bone, shifting the hip's loading forces proximally and simulating a more physiological load transfer that reduces proximal stress shielding caused by conventional stems.^2,5,12 Short stems can be divided into two categories according to their contact points with the bone: 1) medial-to-lateral (ML) stems are flat in the sagittal plane and engage the bone in the coronal plane, while 2) metaphyseal-filling (MF) stems engage the bone in both the coronal and sagittal planes.¹²

Preoperative surgical preparation for THA entails balancing the patient's functional needs, femoral morphology and selecting the most appropriate implant to fit each case. Preoperative radiographic measurements are used to analyze femoral morphology and aid in implant selection via preoperative templating.³ This process relies on radiographic anteroposterior and lateral images to measure different points on the proximal femur and calculate various indices and ratios that reflect femoral morphology. Commonly used measurements are the canal flare index and the canal-calcar ratio, which provide a measurement of the funneling or widening of the intramedullary canal from the calcar to the isthmus and the mid-lesser trochanter to the isthmus, respectively (Fig. 1).^3,13,14 The cortical index and the canal-bone ratio measure the thickness of the cortical bone at the isthmus (Fig. 1).^13,15 Finally, the morphological cortical index measures the relative extramedullary diameter at the calcar in relation to the intramedullary diameter 7 cm distal to the lesser trochanter (Fig. 1).^16,17 These measurements were initially described to assist surgeons in deciding whether to use cemented or uncemented implants, and canal flare index, cortical index, and canal-calcar ratio have all been correlated to bone quality assessment based on the Dorr classification, making these advantageous when dealing with metaphyseal-diaphyseal mismatch.^14,15,18,19

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Fig. 1 A. Radiographic image of a THA with a conventional cementless stem. The solid and dashed horizontal lines to the left of the femur represent the reference points traditionally used for measurement. The arrows within the femur represent the outer cortical diameters and intramedullary canal diameters measured at each point to produce the commonly used indices: Canal Flair Index = A/F, Canal-Calcar Ratio = F/B, Cortical Index = (G-F)/G, Morphological Cortical Index = C/E, Canal-Bone Ratio = F/G. Fig. 1-B. Radiographic image of a THA with a modern, short, metaphyseal-engaging stem. Note the two most distal reference points at 7 and 10 cm distal to the mid-lesser trochanter, respectively, are distal to modern, short, metaphyseal-engaging stems. On the contrary, the three proximal reference points, used by the authors to create the novel indices and ratios proposed, do engage areas of fixation for shorter stems.

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Nonetheless, all the radiographic indices to date are based on morphological landmarks specific to conventional stems, using the isthmus, 10 cm distal to the mid portion of the lesser trochanter, as a reference point.¹³ However, many modern metaphyseal-engaging short stems do not engage the isthmus, making these indices less accurate for implant selection. As such, the purpose of this study was to 1) propose and evaluate a new set of radiographic indices and ratios tailored to modern, shorter, metaphyseal-engaging stems and 2) determine if there is a difference in the relative radiographic performance of modern ML and MF stems.

2 Material and methods

2.2

2.2 Radiographic measurements

A single, fellowship-trained observer assessed preoperative and postoperative radiographs. All radiographs were calibrated for magnification using a 25 mm metallic sizing reference device. Three radiographic reference points were selected based on accepted reference points as reported by Dorr et al. and Noble et al.^3,13 for preoperative measurements: 1) the center of the lesser trochanter (referred to as “Lesser”), 2) 2 cm proximal to the center of the lesser trochanter (referred to as “Proximal”), and 3) 3 cm distal to the center of the lesser trochanter (referred to as “Distal,” Fig. 2). The outer cortical diameter (OCD) and intramedullary canal diameter (ICD) were measured at these three reference points. These six measurements were combined into three cortical indices and three canal-canal ratios.

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Fig. 2 Radiographic image of an osteoarthritic hip. The solid and dashed horizontal lines to the left of the femur represent the reference points at the level of the mid-lesser trochanter, 2 cm proximal and 3 cm distal, selected for the development of the novel indices and ratios. The arrows within the femur represent the outer cortical diameters and intramedullary canal diameters measured at each point to produce the novel indices and ratios: Proximal Cortical Index = (A–B)/A, Lesser Cortical Index = (C–D)/C, and Distal Cortical Index = (E–F)/E, Canal-Canal Ratio 1 = F/D, Canal-Canal Ratio 2 = F/B and Metaphyseal Flair Index = D/B. Both Lesser Cortical Index and Metaphyseal Flair Index were found to be predictors of failed osseointegration for values above 0.3 and below 0.6, respectively.

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Cortical Indices. Cortical indices represent the thickness of the cortical bone at any given level. They generate a value between 0 and 1, where smaller values refer to thinner cortices and larger values refer to thicker cortices.¹³ProximalCorticalIndex=(ProximalOCD−ProximalICD)ProximalOCDLesserCorticalIndex=(LesserOCD−LesserICD)LesserOCDDistalCorticalIndex=(DistalOCD−DistalICD)DistalOCD

Canal–Canal Ratios. Canal–canal ratios represent the distal change in the diameter of the femoral canal. These ratios generate a value between 0 and 1, where smaller ratios indicate a funnel-shaped femoral canal and larger ratios represent a cylindrical-shaped femoral canal.¹³Canal−CanalRatio1=DistalICDLesserICDCanal−CanalRatio2=DistalICDProximalICDCanal−CanalRatio3(MetaphysealFlairIndex)=LesserICDProximalICD

Postoperative radiographs were analyzed for evidence of radiolucencies, which were used as a surrogate for lack of osseointegration based on findings by Cooper et al.⁴ Radiographs were categorized as “positive” or “negative” according to the presence or absence of radiolucencies in all Gruen zones, respectively (Fig. 3).

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Fig. 3 Side-by-side radiographic images showing an example of a well-healed metaphyseal-engaging stem (left) and a different metaphyseal-engaging stem showing radiolucencies in Gruen Zones 1 and 7 (right).²⁹.

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3 Results

3.2

3.2 Radiographic findings

Overall rate of positive radiolucencies was 13.6 % (N = 102) and was significantly greater in ML stems (19.3 %, N = 64) than in MF stems (9.1 %, N = 38, p < 0.001), with an odds ratio (OR) of 2.39 (95 % CI 1.55–3.68). Rates of radiolucencies were significantly different across all stem types (p < 0.001) (Fig. 4). Of note, the Actis stem had the lowest rate of radiolucencies reported, at 0 %, and the ODC Tribute stem had the highest at 35.7 % (Fig. 4).

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Fig. 4 Bar graph illustrating the rate of radiolucencies by implant type at six months after THA. The rate was significantly different between implant types, with the Actis stem standing out for its zero rate of radiolucencies, contrasting the Ovation Tribute stem, which had the highest rate at 35.7 % (p < 0.001).

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3.4

3.4 Accuracy of radiographic measurements

Receiver operating characteristic curve analyses for radiographic findings identified the measurements with the best accuracy of predicting positive radiographic findings at six months were the Lesser Cortical Index and the Metaphyseal Flair Index, regardless of implant type used. Lesser Cortical Index had an 83 % probability (AUC 0.83, p < 0.001) of predicting radiolucencies for values above 0.3, and the Metaphyseal Flair Index had a 73 % probability (AUC 0.73, p < 0.001) of predicting radiolucencies for values below 0.6 (Fig. 5). Of note, the value of Metaphyseal Flair Index is inversely related to the rate of radiographic findings, meaning patients with smaller values for Metaphyseal Flair Index were more likely to have positive radiographic findings. All other significant measurements had AUC below 0.7, making them less reliable predictors of osseointegration.

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Fig. 5 Fig. 5-A. Receiver Operating Characteristic curve for the Metaphyseal Flair Index with an area under the curve (AUC) of 0.73, representing a good accuracy for predicting failed osseointegration at a cut-off of 0.6 determined by the Maximum Vertical Distance (MVD). B. Receiver Operating Characteristic curve for the Lesser Cortical Index with an AUC of 0.83, representing a very good accuracy for predicting failed osseointegration at a cut-off of 0.3 determined by the MVD. C. Receiver Operating Characteristic curve for the Lesser Cortical Index for MF stems (red curve) and ML stems (green curve) with an AUC of 0.75 and 0.93, respectively. The Lesser Cortical Index showed excellent prediction of failed osseointegration for ML stems at a cut-off of 0.3 determined by the MVD.

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In addition, ROC curve analysis demonstrated the Lesser Cortical Index to be the most accurate predictor of positive radiographic findings between implant groups, where the Lesser Cortical Index for ML stems had a 93 % probability (AUC 0.93, p < 0.001) of positive radiographic findings for values above 0.3. Whereas the Lesser Cortical Index for MF stems had a 75 % probability (AUC 0.75, p < 0.001) of positive radiographic findings for values above 0.3 (Fig. 5). The AUC for the Distal Cortical Index for the MF stems was not significantly different, rendering this measurement a poor predictor for differentiating implant groups and the association with osseointegration.

4 Discussion

Existing radiographic measurements for femoral morphology that aid in THA implant selection do not accurately reflect the landmarks necessary for selecting modern, shorter, cementless, metaphyseal-engaging stems. We developed six new indices and ratios using standardized and widely accepted radiographic reference points from Noble and Dorr.^3,13 We analyzed their capacity to help the arthroplasty surgeon in implant selection. Simultaneously, we compared short-term radiographic evidence of osseointegration between MF and ML-type short stems and determined which novel radiographic measurements better predicted osseointegration.

We had a slightly higher rate of MF stems (55.7 %) than ML stems (44.3 %). However, demographic characteristics were balanced among our population except for age, where MF patients were, on average, three years older than those with ML stems. This reflected greater utilization of MF stems in older patients over the study period as newer stem designs became available. Nonetheless, this allowed for a fair comparison between MF and ML stems.

Our cohort had a pooled rate of 13.6 % positive radiographic findings (i.e., radiolucencies) six months after THA. This is comparable to other publications looking at osseointegration in conventional tapered stems.²³ However, there is limited literature comparing radiolucency rates between multiple short stems. In this cohort, ML stems had 2.3 higher odds of developing radiolucencies than MF stems. A previous study by Reddy et al. associated ML stems with a higher risk of loosening when compared to MF stems.²⁴ In contrast, Grant et al. reported a higher subsidence rate in MF stems than in ML stems.²⁵ Rates of positive radiographic findings were significantly different across all implant types, with the Actis stem having the lowest rate, at 0 %. This excellent performance by the Actis stem is echoed by the 2023 American Joint Replacement Registry Annual Report and a previous publication comparing it to another short stem.^26,27 On the contrary, rates for all other stems were notably higher than those reported in other single-stem publications.^28–30

Metaphyseal filling stems had consistently larger Proximal Cortical Index, Lesser Cortical Index, and Distal Cortical Index values, indicating that patients who received these stems had thicker cortices at all three reference points. Nonetheless, MF stems had lower rates of radiolucencies, demonstrating MF stems may provide a better option for patients with ticker femoral cortices. Larger values for Proximal, Lesser, and Distal Cortical Indices were significantly associated with radiolucencies at six months, meaning patients with thicker cortices at all reference points had higher rates of positive radiographic findings. Similarly, a smaller Metaphyseal Flair Index, representing a more funneled canal proximal to the lesser trochanter, and a larger Canal-Canal Ratio 1, representing a more cylindrical canal distal to the lesser trochanter, were related to higher rates of radiolucent findings. The latter findings are comparable to Cooper et al., who found smaller values of the older canal flare index (i.e., “stovepipe” proximal femoral morphology), which were more predictive of failed osseointegration.⁴ Further analysis determined that when the rate of radiolucencies within MF and ML stems was compared and associated with our novel radiographic measurements, Lesser Cortical Index and Distal Cortical Index were significantly larger in patients with radiolucencies in both groups.

We implemented ROC curve analysis to determine the Lesser Cortical Index and Distal Cortical Index's usability as preoperative measurements to predict the development of radiolucencies at six months and help guide implant selection. The Lesser Cortical Index outperformed all other measurements in predicting radiolucencies as a surrogate for predictable osseointegration. More specifically, for Lesser Cortical Index values above 0.3, there is an 83 % probability of accurately predicting radiolucencies for all short stems and a 93 % and 75 % probability for ML and MF stems, respectively. Consequently, arthroplasty surgeons may benefit from applying this index in their preoperative planning, where patients with Lesser Cortical Index values above 0.3 are at a higher risk of radiolucencies (less predictable osseointegration) at six months. Furthermore, they might defer from choosing an ML stem for these patients due to a higher risk of developing radiolucencies (93 % probability, Fig. 6).

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Fig. 6 Graphical illustration of the relationship between the Lesser Cortical Index and the rate of radiolucencies across implant groups. Vertical bars represent the radiolucency rate for MF and ML stems. The blue circle and orange triangle represent the mean Lesser Cortical Index measured for cases with positive and negative radiolucencies, respectively. The 0.3 cut-off for Lesser Cortical Index is represented by the horizontal blue line across the graph. The cortical thickness diagram provides a visualization of the expected increase in cortical thickness at the level of the lesser trochanter as the value of Lesser Cortical Index increases.

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Similarly, the Metaphyseal Flair Index was a good general predictor of osseointegration for values below 0.6, where funnel-shaped proximal femoral canals were more likely to present radiolucencies at six months. These findings point to the utility of novel measurements tailored to short stems, as the risk of overstuffing the femoral canal and altering the transfer of forces from the stem to the more distal bone, causing radiolucencies and lack of osseointegration proximally, may be avoided by accounting for femoral cortical thickness and shape at reference points significant to short stems.

As this was a retrospective study, several limitations were introduced to the data collection process. Some selection bias may have been introduced by methodological factors of this study, such as being a single-center study, the inclusion of surgeries performed by multiple fellowship-trained surgeons, the inclusion of multiple different implant designs, and thus, the exclusion of a large portion of the available cohort owing to the interest in a predefined set of femoral implants and loss to follow-up. However, some of these methodological decisions also introduce necessary generalizability to the findings (i.e., different implants and surgeons) and a more consistent cohort (i.e., the inclusion of only a subset of implants) that we believe confidently represent ML and MF stems. Additionally, although institutional protocols exist to standardize radiographic imaging, we could not control image quality and relied on imaging available during review. Similarly, rotational differences between radiographs and potential variations in Proximal OCD and ICD measurements due to anatomical variations can cause an alteration of measurement fidelity, introducing some unmeasurable limitations to the precision of the measurements calculated which cannot be controlled for in the broader applicability of the indices in this study. Furthermore, the radiographic reporting of radiolucencies in this patient population was not correlated with clinical outcomes (i.e., revision surgery rate and thigh pain), limiting the understanding of their clinical significance. However, due to the low incidence of fracture and aseptic loosening in this cohort, radiolucent lines were analyzed as a surrogate for stem-femur mismatch, as suggested by Cooper et al.,⁴ which is often used as a surrogate for implant osseointegration. Similarly, radiographic measurements and evaluations were done by a single observer, introducing a potential for bias and error in the calculations. This study also collected radiographic measurements on short-term outcomes (i.e., minimum six months follow-up), unfortunately forcing a lack of long-term evaluation of these measurements. As such, these novel measurements would benefit from future research focusing on their validation to 1) aid in preoperative planning and stem selection based on femoral morphology, 2) predict osseointegration in short and long-term follow-up and more diverse cohorts (i.e., minimum one year follow-up and multi-center study) and 3) using radiolucent lines as a surrogate for stem-femur mismatch together with these novel measurements.

5 Conclusion

This is the first study to develop, implement, and analyze a range of novel radiographic measurements tailored to modern, shorter, metaphyseal-engaging femoral stems and to identify a predictive association between these novel measurements and short-term osseointegration. The proposed Metaphyseal Flair Index and Lesser Cortical Index show promise for use preoperatively in predicting osseous integration with different femur morphology and stem design. Furthermore, values above 0.3 for the Lesser Cortical Index can assist surgeons in selecting MF stems over ML stems for more predictable osseointegration, as evidenced by reduced radiolucencies. Our findings point to thicker cortices, more funneled proximal canals, and more cylindrical distal canals as predictors of radiolucent zones, underscoring the importance of accurate size selection for short cementless stems. Additionally, this study demonstrates that short modern MF stems have more predictable osseous integration when compared to modern ML stems, with the Actis stem having the most predictable osseointegration, as evidenced by the lack of radiolucencies in this cohort.

CRediT authorship contribution statement

Catalina Baez: Methodology, Formal analysis, Writing – original draft, Visualization. Blane Kelly: Conceptualization, Investigation, Data curation, Writing – original draft. Sunny Trivedi: Investigation, Data curation, Writing – original draft. Jeff Dela Cruz: Investigation, Data curation, Writing – original draft. Justin Deen: Conceptualization, Writing – review & editing. Chancellor F. Gray: Conceptualization, Methodology, Writing – review & editing. Hernan Prieto: Conceptualization, Writing – review & editing. Luis Pulido: Conceptualization, Writing – review & editing. MaryBeth Horodyski: Methodology, Formal analysis, Writing – review & editing. Hari Parvataneni: Conceptualization, Methodology, Writing – review & editing, Supervision.

Guardian/patient consent

As this was a retrospective study with no direct contact or intervention on the subjects being studied, the need for patient consent was waived by the Institutional Review Board under approval No. 202003271. This approval document was uploaded as the “Ethical statement” accompanying this submission. Similarly, no underage (<18y) subjects were enrolled in this study, circumventing the need for patient parent/guardian consent for participation for any included subject.

Funding sources

The study team did not receive any direct funding to perform this research other than the data collection voucher recognized in the “Acknowledgements” section of this submission.