2.4.2. Insertion

The custom short stem design is characterized by mimicking the curvature of the lateral side of the proximal femur (Figure 5A). This lateral widening requires a new implantation method to achieve femoral reaming, which consists of gradually opening the cavity using calibrated elements similar to the stem until the appropriate size is achieved for insertion, while respecting the greater trochanter and the gluteal muscles. This technique has been called "round the corner" and is possible due to the absence of the distal part of the stem [2,9]. "Round the corner" requires that the reamers and final implant are first inserted in the varus position and then progressively tilted into the correct alignment while descending the femoral metaphysis (Figure 6).

This technique facilitates the use of minimally invasive approaches such as Micro-Hip [49], but precludes the use of intramedullary guides and may also result in a varus position when the tip of the stem touches the lateral side of the femur, contributing to a possible fracture, so the use of fluoroscopy during insertion is advisable [50].

**Figure 6. "**Round the corner" technique. **Figure 6.** "Round the corner" technique. **Figure 6. "**Round the corner" technique.

### 2.4.3. Implantability 2.4.3. Implantability 2.4.3. Implantability

The designers, despite having the virtual model, often do not consider in the design process the osteotomy and the insertion method, key aspects that determine whether the customized prosthesis is implantable or not. Consequently, a methodology is proposed to study implantability by ensuring that the prosthesis adapts to the canal and its insertion is possible. From a geometric point of view, to use the "round the corner" technique, the limits of the implant sections must be projections of the implantation curve on the planes used in the cavity analysis (Figure 5), due to the rotation performed to place the stem in the correct alignment (Figure 6). The orthogonal projection of the implantation or osteotomy curve can be of two types: the first consists of projecting the I section on the oblique planes (S1); in the second, the I curve is projected on the II plane, and the result is projected on the III plane and so on until reaching the IV plane (S2). Interpolating the generated curves, two solids are formed; however, not only these bodies must be evaluated, but the intersection (S3) and union (S4) of both must also be included in the implantability analysis. These four solids represent the **constraint of the implantation section** (Figure 7A). The designers, despite having the virtual model, often do not consider in the design process the osteotomy and the insertion method, key aspects that determine whether the customized prosthesis is implantable or not. Consequently, a methodology is proposed to study implantability by ensuring that the prosthesis adapts to the canal and its insertion is possible. From a geometric point of view, to use the "round the corner" technique, the limits of the implant sections must be projections of the implantation curve on the planes used in the cavity analysis (Figure 5), due to the rotation performed to place the stem in the correct alignment (Figure 6). The orthogonal projection of the implantation or osteotomy curve can be of two types: the first consists of projecting the I section on the oblique planes (S1); in the second, the I curve is projected on the II plane, and the result is projected on the III plane and so on until reaching the IV plane (S2). Interpolating the generated curves, two solids are formed; however, not only these bodies must be evaluated, but the intersection (S3) and union (S4) of both must also be included in the implantability analysis. These four solids represent the **constraint of the implantation section** (Figure 7A). The designers, despite having the virtual model, often do not consider in the design process the osteotomy and the insertion method, key aspects that determine whether the customized prosthesis is implantable or not. Consequently, a methodology is proposed to study implantability by ensuring that the prosthesis adapts to the canal and its insertion is possible. From a geometric point of view, to use the "round the corner" technique, the limits of the implant sections must be projections of the implantation curve on the planes used in the cavity analysis (Figure 5), due to the rotation performed to place the stem in the correct alignment (Figure 6). The orthogonal projection of the implantation or osteotomy curve can two types: the first consists of projecting the I section on the oblique planes (S1); in the second, the I curve is projected on the II plane, and the result is projected on the III plane and so on until reaching the IV plane (S2). Interpolating the generated curves, two solids are formed; however, not only these bodies must be evaluated, but the intersection union (S4) of both must also be included in the implantability analysis. These four solids represent the **constraint of the implantation section** (Figure 7A).

This technique facilitates the use of minimally invasive approaches such as MicroHip [49], but precludes the use of intramedullary guides and may also result in a varus position when the tip of the stem touches the lateral side of the femur, contributing to a possi-

This technique facilitates the use of minimally invasive approaches such as MicroHip [49], but precludes the use of intramedullary guides and may also result in a varus position when the tip of the stem touches the lateral side of the femur, contributing to a possi-

ble fracture, so the use of fluoroscopy during insertion is advisable [50].

ble fracture, so the use of fluoroscopy during insertion is advisable [50].

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**Figure 7.** Assessment of implantability (**A**) without and (**B**) with the trabecular bone. **Figure 7.** Assessment of implantability (**A**) without and (**B**) with the trabecular bone. **Figure 7.** Assessment of implantability (**A**) without and (**B**) with the trabecular bone.

However, the **cavity constraint** should be included in the analysis. As in the previous case, the adjusted curves obtained from the study of the femoral canal are interpolated, forming a solid that approximates the patient's cavity; therefore, it is necessary to rectify However, the **cavity constraint** should be included in the analysis. As in the previous case, the adjusted curves obtained from the study of the femoral canal are interpolated, forming a solid that approximates the patient's cavity; therefore, it is necessary to rectify However, the **cavity constraint** should be included in the analysis. As in the previous case, the adjusted curves obtained from the study of the femoral canal are interpolated, forming a solid that approximates the patient's cavity; therefore, it is necessary to rectify areas of overestimation, which invade the cortical part of the femur, intercepting it with the trabecular bone or subtracting the cortical one, achieving, as a result, the maximum volume of the customized stem (Figure 7A). Both constraints must be considered to ensure the implantability of the prosthesis; therefore, the solids, which are a physical representation of the constraints generated by the patient's cavity and the implantation section, have to

be intercepted. As a result, four regions are produced, R1, R2, R3 and R4, which are a consequence of the intersection of S1, S2, S3 and S4 with the cavity constraint (Figure 7A). tion of the constraints generated by the patient's cavity and the implantation section, have to be intercepted. As a result, four regions are produced, R1, R2, R3 and R4, which are a consequence of the intersection of S1, S2, S3 and S4 with the cavity constraint (Figure 7A).

areas of overestimation, which invade the cortical part of the femur, intercepting it with the trabecular bone or subtracting the cortical one, achieving, as a result, the maximum volume of the customized stem (Figure 7A). Both constraints must be considered to ensure the implantability of the prosthesis; therefore, the solids, which are a physical representa-

Regarding the cavity constraint, the question may arise as to why the trabecular bone, which contains exact information about the patient's cavity, is not used directly; this is because, when intercepting the trabecular bone with the constraint of the implantation section, the result invades the lesser trochanter, which, according to the study of Solórzano et al. [18], is a moderately critical area of the proximal femur (Figure 7B). Therefore, fitted curves that adapt to the femoral canal, and do not invade the lesser trochanter, allow proper implant design, avoiding periprosthetic fractures. Regarding the cavity constraint, the question may arise as to why the trabecular bone, which contains exact information about the patient's cavity, is not used directly; this is because, when intercepting the trabecular bone with the constraint of the implantation section, the result invades the lesser trochanter, which, according to the study of Solórzano et al. [18], is a moderately critical area of the proximal femur (Figure 7B). Therefore, fitted curves that adapt to the femoral canal, and do not invade the lesser trochanter, allow proper implant design, avoiding periprosthetic fractures.

To test the implantability, the four regions and the cortical part already included in the osteotomy for GC1 (Figure 8B) and GC2 (Figure 8C) were fabricated to imitate the "round the corner" technique, certifying that the regions enter through the I section and fit the cavity properly. To test the implantability, the four regions and the cortical part already included in the osteotomy for GC1 (Figure 8B) and GC2 (Figure 8C) were fabricated to imitate the "round the corner" technique, certifying that the regions enter through the I section and fit the cavity properly.

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**Figure 8.** Prototype fabrication. (**A**) Planning using Ultimaker Cura®. PLA prototypes with their respective STLs for (**B**) GC1 and (**C**) GC2. Evidence of experiments performed for (**D**) GC1 and (**E**) GC2 demonstrating the insertion of regions 1, 2 and 3 (left), but not 4 (right). **Figure 8.** Prototype fabrication. (**A**) Planning using Ultimaker Cura®. PLA prototypes with their respective STLs for (**B**) GC1 and (**C**) GC2. Evidence of experiments performed for (**D**) GC1 and (**E**) GC2 demonstrating the insertion of regions 1, 2 and 3 (left), but not 4 (right).

Therefore, using fused material deposition printing, PLA prototypes were produced from the STL files of the solids, which were laminated in Ultimaker Cura 4.8.0® (Ultimaker, Geldermalsen, Netherlands; Figure 8A) and manufactured using the Ender 3 Pro® (Creality, Shenzhen, China) printer. Therefore, using fused material deposition printing, PLA prototypes were produced from the STL files of the solids, which were laminated in Ultimaker Cura 4.8.0® (Ultimaker, Geldermalsen, Netherlands; Figure 8A) and manufactured using the Ender 3 Pro® (Creality, Shenzhen, China) printer.

The results showed, for both geometric cases, that regions 1, 2 and 3 are implantable solids; therefore, the customized stem was designed from them; however, region 4 is not implantable because it did not enter through the cavity (Figure 8D,E). Now, the choice of The results showed, for both geometric cases, that regions 1, 2 and 3 are implantable solids; therefore, the customized stem was designed from them; however, region 4 is not implantable because it did not enter through the cavity (Figure 8D,E). Now, the choice of two geometric cases with different femoral morphology makes sense since it gives reliability to the conclusions obtained from experimentation; however, to generalize this behavior, further testing with other patients is needed. Moreover, to emphasize that this study is possible thanks to the new methodology proposed to study the femoral cavity since, had the conventional technique been used, the restriction of the implantation section would be

an elliptical cylinder. As a result, the implantable solid would not adapt to the lateral side of the femur, which would impair its biomechanics after surgery. the lateral side of the femur, which would impair its biomechanics after surgery. 2.4.4. Stem

two geometric cases with different femoral morphology makes sense since it gives reliability to the conclusions obtained from experimentation; however, to generalize this behavior, further testing with other patients is needed. Moreover, to emphasize that this study is possible thanks to the new methodology proposed to study the femoral cavity since, had the conventional technique been used, the restriction of the implantation section would be an elliptical cylinder. As a result, the implantable solid would not adapt to

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### 2.4.4. Stem The implantable regions, because they adequately fit and fill the patient's cavity and

The implantable regions, because they adequately fit and fill the patient's cavity and enter through the implantation section, were the stem of the customized short implant. However, because of the Boolean operations performed, they were not uniform (Figure 8), hindering bone ingrowth; therefore, they were smoothed, preserving their shape using the Meshmixer® smooth tool with a smoothing scale of 50, and, to facilitate its insertion through the femoral canal, the edge of the VI section was rounded by 5 mm. enter through the implantation section, were the stem of the customized short implant. However, because of the Boolean operations performed, they were not uniform (Figure 8), hindering bone ingrowth; therefore, they were smoothed, preserving their shape using the Meshmixer® smooth tool with a smoothing scale of 50, and, to facilitate its insertion through the femoral canal, the edge of the VI section was rounded by 5 mm.

## 2.4.5. Neck and Receiving Taper 2.4.5. Neck and Receiving Taper Mimicking European standards, the custom stem taper was 12/14 since, according to

Mimicking European standards, the custom stem taper was 12/14 since, according to the study by Morlock et al. [51], this receiving taper is the most commonly used in that continent. The 12/14 model is defined by a proximal diameter of 12 mm, distal of 14 mm and a height of 20 mm, resulting in a taper angle of 5◦43'30". the study by Morlock et al. [51], this receiving taper is the most commonly used in that continent. The 12/14 model is defined by a proximal diameter of 12 mm, distal of 14 mm and a height of 20 mm, resulting in a taper angle of 5°43'30''. To model the taper, a plane perpendicular to the femoral neck plane was defined and,

To model the taper, a plane perpendicular to the femoral neck plane was defined and, to preserve the neck–shaft angle in the design, it was rotated (90-MAS)◦ counterclockwise with respect to the Y-axis. In addition, following the recommendation of Wen-Ming et al. [24], it was placed at the middle of the sphere, which approximates the femoral head, obtaining an oblique plane where the sketch of the 12 mm circumference was drawn. To maintain the height of the cone, a plane 20 mm below the oblique plane was positioned and the sketch of the 14 mm circumference was drawn following the direction of the femoral neck axis. For the neck, the initial curve of the stem was needed, which was obtained by projecting the first section onto the osteotomy plane. This whole process is illustrated in Figure 9A. to preserve the neck–shaft angle in the design, it was rotated (90-MAS)° counterclockwise with respect to the Y-axis. In addition, following the recommendation of Wen-Ming et al. [24], it was placed at the middle of the sphere, which approximates the femoral head, obtaining an oblique plane where the sketch of the 12 mm circumference was drawn. To maintain the height of the cone, a plane 20 mm below the oblique plane was positioned and the sketch of the 14 mm circumference was drawn following the direction of the femoral neck axis. For the neck, the initial curve of the stem was needed, which was obtained by projecting the first section onto the osteotomy plane. This whole process is illustrated in Figure 9A.

**Figure 9.** Neck and receiving taper (**A**) sketches and (**B**) solid. (**C**) Comparison between implanted and intact femur for GC1 and GC2.

Based on the drawn curves, which form the neck and the receiving taper, a solid was obtained and integrated into the stem through the Boolean union operation; to avoid stress concentration, the edges of the curves were rounded as shown in Figure 9B. The described process was repeated for each region (R1, R2 and R3) to obtain the final custom stem designs (V1, V2 and V3). Once the design of the short stem has been completed with the described methodology that considers the patient's features, such as his anatomy, and those that depend on the surgery—osteotomy and insertion—the adjustment, filling and implantability of the prosthesis are guaranteed. However, to carry out the design, elements of the femoral morphology study were used, such as the LT plane, the MAS, the femoral neck axis and plane and the sphere that fits the femoral head, thanks to which it was possible to restore the neck–shaft angle, anteversion and offset of each geometric case, as visualized in Figure 9C. Therefore, none of the proposed designs is expected to fail due to muscle imbalance, modification of the range of motion or impingement with the acetabulum. Another point in favor of this technique is its simplicity, since the designer does not need to manually define the implant–bone contact zones because the program created provides the curve that best fits the bone section guaranteeing primary stability, thus avoiding human error in the process. The next step was to perform the finite element analysis (FEA) to select which of the three options is the best stem for each geometric case.
