Effects of Ilial and Ischial Osteotomy Angles on the Structural Pelvic Anatomy in Triple and 2.5 Pelvic Osteotomy Using 3D-printed Bone Models

Abstract

Objective

To describe the effects of varying ilial and ischial osteotomy angles on the structural anatomy of the pelvis in triple (TPO) and 2.5 pelvic osteotomy (2.5 PO) models regarding femoral head coverage (FHC), interischiatic tuberosity distance (ITD), and displacement of the caudal ilial fragment.

Study Design

Descriptive, anatomical study on three-dimensionally printed, pelvic models.

Methods

A total of 10 identical three-dimensionally printed polyethylene models replicating a non-dysplastic, pelvic specimen of a 20 kg dog were used. Three types of ilial osteotomy were compared in TPO models and combined with two types of ischial osteotomy in 2.5 PO models. Computed tomography (CT) scans were performed, and measurements were obtained using multiplanar reconstructed (MPR) images and thick-slab mean rendering.

Results

The ilial osteotomy perpendicular to the pelvic floor (TC) provided greater and more equally distributed FHC in TPO and 2.5 PO models. Combination of TC ilial osteotomy with a sagittal ischial osteotomy (LO) resulted in the least amount of displacement of the caudal ilial fragment in 2.5 PO. Medially angled ischial osteotomy (MO) resulted in a high degree of lateralization of the caudal ilial segment.

Conclusions and Clinical Relevance

Ilial osteotomy perpendicular to the pelvic floor (TC) may potentially reduce bending and shearing forces on implants and bone in TPO and 2.5 PO surgery. Sagittal orientation of the partial ischial osteotomy in 2.5 PO appears to be imperative to achieve reduction of the ilial osteotomy site. Clinical relevance of these findings would need to be investigated further.

Keywords

TPO - triple pelvic osteotomy - 2.5 pelvic osteotomy - hip dysplasia - dog - 3D-printed bone model

Introduction

Pelvic osteotomies are frequently used to treat immature dogs with hip dysplasia by improving femoral head coverage (FHC) through rotation of the acetabulum. The most commonly performed techniques are the triple pelvic osteotomy (TPO)[1] and its derivatives, the double pelvic osteotomy (DPO)[2] and the 2.5 pelvic osteotomy (2.5 PO).[3] While the DPO was developed to increase stability of the pelvis by omitting the ischial osteotomy, the 2.5 PO introduced a mono-cortical ischial cut to maintain stability via a bony connection across the ischial table while at the same time reducing the force necessary for acetabular rotation compared with DPO.[4] However, there are still no published case series on 2.5 PO to validate this hypothesis.

Besides improving FHC, rotation of the acetabular segment also has some undesirable effects on the pelvic anatomy leading to potential complications, with screw loosening historically being by far the most common.[5] [6] [7] [8] With the introduction of specifically modified, interlocking implants the incidence of screw loosening could be reduced significantly.[9] [10] Nevertheless, in these cases a new type of complication has been observed, namely, an en bloc pullout with fracture of the caudal ilium, supposedly shifting the weakest link from the bone–implant interface to the bone structure itself.[2] [10] [11]

Complications after pelvic osteotomies like implant failure or bone fractures result from a combination of elastic forces caused by the resistance of the rotated bone and soft tissue and dynamic forces caused by weight bearing and movement. It therefore seems prudent, independent of the type of implants used, to minimize strain on bone and implants by avoiding strong bending and shearing forces after rotation of the acetabular segment.

Hence, the axis of rotation ideally should be identical to the axis formed by the intersection of the cranial and caudal plane of the plate, which after rotation would result in a perfect congruency of the implant and the two parts of the osteotomized ilium.

In the TPO, the axis of rotation is principally defined by the plate, since the acetabular segment is freely moveable and only restricted by soft tissue. Therefore, in the TPO, the main disruption of normal pelvic architecture occurs at the level of the acetabulum and the ischial osteotomy and has previously been described by changes in interischiatic tuberosity distance (ITD).[12]

In contrast, in the 2.5 PO the axis of rotation coincides with the partial ischial osteotomy which lies more ventrally and medially compared with the axis formed by a classically designed TPO plate. Rotation therefore will result in an increased incongruency of bone and plate at the ilial osteotomy site. To correct this incongruency, increased bending forces on the caudal ilial segment are required to bring the bone in alignment with the plate. Something similar applies for DPO in which the fulcrum of rotation is located within the pelvic symphysis and the ischial table.[13] [14]

Graehler et al have demonstrated that the angle of the ilial cut has a significant influence on the degree of disruption of the normal pelvic anatomy in TPO. The authors found that an ilial osteotomy perpendicular to the long axis of the animal body (which in their study equaled the 30-degree ilial osteotomy) significantly reduces the amount of lateralization of the acetabular segment, the degree of acetabular version, and the ITD after rotation, potentially making it a better option for the ilial cut. It still was not recommended for all cases of TPO surgery due to the potential lack of bone stock to secure the plate in the caudal ilial segment.[12]

Different recommendations exist in the literature about the reference points for and the optimal angle of the ilial osteotomy. The original paper on the TPO by Slocum and Slocum describes introducing a Kirschner pin resting on the dorsum of the lateral prominence of the tuber ischii, passing tangentially to the dorsal acetabular rim and ending on the lateral part of the ilial wing on the junction of its ventral and middle third. The authors define this as the long axis of the body of the ilium. The osteotomy is orientated perpendicular to this axis at the caudal junction of the sacrum and the body of the ilium.[1] Other authors define a line from the center of the acetabulum along the center of the body of the ilium ending on the midpoint of the cranial border of the ilial wing as the long axis of the ilium and perform the perpendicular cut respectively. This results in a more caudally angled ilial osteotomy compared with the osteotomy and Slocum.[12] [15]

In the TPO, different ilial osteotomy angles result in different axes of rotation, consequently changing the orientation of the rotated acetabulum. This can potentially lead to an unevenly distributed FHC gain. A study by Moores and colleagues demonstrates similar odds for full weight loading in the cranial and caudal third of the acetabulum.[16] An equal distribution of FHC gain after pelvic osteotomy procedures therefore appears to be desirable to achieve a result that anatomically and functionally approximates a normal hip joint.

The purpose of this study is to investigate the effects of varying ilial and ischial osteotomy angles on the structural anatomy of the pelvis in triple and 2.5 PO in a three-dimensionally printed polyethylene model.

Three different ilial osteotomies in the TPO surgery, their effect on ischial tuberosity displacement, and gain in FHC were examined. In the second stage, the degree of lateralization of the caudal ilial fragment and the increase in FHC were compared for the three ilial osteotomy angles and two types of mono-cortical ischial osteotomy in 2.5 PO.

A further objective of this study is to compare the different ilial osteotomies regarding the distribution of FHC gain between the cranial and caudal half of the femoral head in both TPO and 2.5 PO surgeries.

Material and Methods

In this study 10 identical three-dimensionally printed polyethylene models replicating a non-dysplastic, pelvic specimen of a 20 kg dog were used. In the TPO group, a right-sided surgery was performed on three models using three different ilial osteotomy angles. A right-sided 2.5 PO procedure was performed on six models. These six models were randomly divided in groups of two, for the three different ilial osteotomy angles, each combined with two types of a partial ischial osteotomy. One model was left unchanged to be used as the zero reference.

The pubic ostectomy was performed identically in all PO models. The ilial osteotomies were executed in three different angles, the ilial osteotomy by Slocum (SC),[1] a more caudally angled osteotomy (BC)[15] equivalent to the 0-degree ilial osteotomy of Graehler et al,[12] and an osteotomy perpendicular to the pelvic floor (TC), similar to the 30-degree osteotomy by Graehler et al.[12] To increase bone stock for the caudal plate without potentially cutting into the sacroiliac joint, the latter cut was positioned more cranially and performed in an inverted L-shape fashion ([Fig. 1]).

Fig. 1 Three different ilial cuts and their respective lines of reference (TC, Tommasini cut; SC, Slocum cut; BC, Bojrab cut).

In the TPO models, the bi-cortical, ischial cut was directed in a sagittal plane ending at the lateral limit of the obturator foramen. A commercial Slocum-type 20-degree TPO plate (Gruppo Bioimpianti S.R.L., Milan, Italy) was applied to the ilium. The lower edge of the plate was levelled along the ventral ilial border of the distal segment. The models were mounted on a holding device made of polystyrene and an appropriately sized, right femoral bone before performing a computed tomography (CT) scan to measure FHC and interischiatic tuberosity distance ([Fig. 2]).

Fig. 2 Zero-model mounted on femoral head for CT scanning.

In each group of the 2.5 PO models, two different mono-cortical, ischial osteotomies were performed. One cut was positioned, equivalent to the TPO models, in a sagittal plane at the lateral limit of the obturator foramen (LO).[3] The second type of ischial osteotomy was started from the same point at the caudal ischium but was aimed toward the middle of the obturator foramen (MO) ([Fig. 3]). The partial osteotomy was simulated on the dorsal aspect of the ischium using a soldering iron and the acetabular segment was rotated 20 degrees. After the heated plastic had cooled down, the position of the rotated segment remained fixed. The models were then placed on the holding device and a CT scan to evaluate the bone contact area between the two cut surfaces of the ilial osteotomy was performed.

Fig. 3 Two different angles for the ischial cut aimed toward the middle (MO) and lateral obturator foramen (LO).

In a second stage, an identical 20-degree Slocum-type TPO plate was applied to the models. First, the plates were fixed to the caudal ilial segment with the ventral border of the plate being in alignment with the ventral border of the ilium. The width of the gap between the cranial part of the plate and the ilial cortex was measured using digital calipers. Then the plate was screwed onto the cranial ilium, thereby reducing the gap between the cortex and the plate until the implant was flush with the bone, and a second scan was performed to measure FHC.

The CT scans of all models were performed using a Siemens SOMATOM go.Now 16-slice CT scanner. The dataset of each pelvic model was processed using a dedicated DICOM viewer software (Horos version 3.0.0, Horosproject.com) and assessed using the three-dimensional MPR viewer function to choose the most representative cut plane for each bone measurement.

Assessment of Positional Accuracy

Positional accuracy between the different models was controlled by measuring the obturator foramen contralateral to the ilial cut in two defined positions. The cranial transverse diameter was a line tangential to the caudal acetabular rims. The longitudinal diameter was a line perpendicular to the previous, passing through the ilio-pectineal eminence ([Fig. 4]).

Fig. 4 Control of model position by measuring the contralateral obturator foramen.

Assessment of Interischiatic Tuberosity Distance

The distance between the lateral limits of the ischiatic tuberosities was measured in the TPO models and the zero-model on a dorsal CT image.

Assessment of Bone Contact Area at the Ilial Osteotomy

The bone contact area at the ilial osteotomy was measured on transverse thick-slab mean rendered MPR images. The image plane for measurements was obtained with a longitudinal line passing through the middle of the ilium body and tangential to the dorsal acetabular rim. At the level of the ilial cut the image plane was perpendicular to the ilial body ([Fig. 5A]). On these images the cranial and caudal cut surfaces appeared superimposed with variable degrees of overlap depending on the type of osteotomy ([Fig. 5B]). The caudal osteotomy area and the overlapping area were measured, and the percentage of overlap was calculated.

Fig. 5 (A, B) Superimposition method for the assessment of the bone contact area at the level of ilial osteotomy: control of the correct orientation of the multiplanar reformatted CT images. cd, caudal surface of the ilial osteotomy; cr, cranial surface of the ilial osteotomy.

Assessment of Femoral Head Coverage

FHC was measured on the dorsal MPR images with the dorsal plane parallel to a line tangential to the cranioventral margin of the acetabulum and to the most caudal apex of the ischium. Then the pelvic floor was volumized using thick-slab mean rendering that allowed the distinction of the femoral head from the acetabular rim similar to radiographic superimposition ([Fig. 6A]).

Fig. 6 Superimposition method for the assessment of femoral head coverage (FHC): control of the correct orientation of the multiplanar reformatted CT images (A). The femoral head area is divided in a cranial and caudal half and the respective areas of FHC are manually traced (B).

On the dorsal thick-slab mean rendered images of the femoro-acetabular joint a circular region of interest (ROI) was traced on the femoral head circumference and the total femoral head area was recorded. The gain in FHC was calculated from the zero-reference model. Then the area was divided in the transverse plane into a cranial and a caudal half. In both sections the area of the femoral head covered by the acetabulum was manually traced and recorded ([Fig. 6B]). Finally, the distribution of FHC gain between the caudal and cranial half of the femoral head was calculated and expressed by a ratio.

Results

The interischiatic tuberosity distance in the triple pelvic osteotomy models showed a lateral displacement of the ipsilateral tuber ischiadicum in the BC and SC model. The degree of displacement was highest in the BC model. Only the TC model showed a displacement in a medial direction ([Table 1]).

Table 1 Interischiatic tuberosity distance (ITD) after rotation in TPO models

TPO model	Zero	TC	SC	BC
ITD (mm)	122	115	125	145
Ischial displacement (mm)		7	3	20

Abbreviation: TPO, triple pelvic osteotomy.

In the TPO models, FHC gain was largest in the TC model and lowest in the BC model. The TC model showed an increase of 65.8% of the original FHC (zero model), the SC model a 51.1% gain and the BC model a gain of only 15.6%. The distribution between cranial and caudal half of the femoral head was most balanced in the TC model (1:1.2). The more caudally angled the ilial osteotomy, the more the proportion of coverage shifted in favor of the caudal femoral head, with the BC model showing FHC gain exclusively in the caudal half ([Table 2]).

Table 2 Femoral head coverage (FHC), gain of FHC after rotation, and gain ratio between cranial (cr) and caudal (cd) half of the femoral head

TPO model	Zero/cr	Zero/cd	TC/cr	TC/cd	SC/cr	SC/cd	BC/cr	BC/cd
FHC (mm2)	126	105	195	188	168	181	125	142
Gain FHC (mm2)			69	83	42	76	−1	37
Ratio			1:1.2		1:1.8		1:37

Abbreviation: TPO, triple pelvic osteotomy.

In the 2.5 PO models, the area of superimposition representing the bone contact area between the ilial osteotomy surfaces was largest in the TC/LO model. In the LO group, bone contact area decreased from the TC to the SC and further in the BC model, which achieved only 50% of the bone contact area of the TC model ([Table 3]).

Table 3 Bone contact area at ilial osteotomy after rotation

2.5 PO model	TC/LO	SC/LO	BC/LO	TC/MO	SC/MO	BC/MO
Total caudal surface area of ilial osteotomy (mm2)	330	320	290	360	359	330
Area of superimposition (mm2)	220	160	110	95	35	75
Bone contact area (%)	66.67	50.00	37.93	26.39	9.75	22.73

All models with a MO partial ischial osteotomy had much smaller bone contact areas compared with the LO group. In these models, reduction of the gap between plate and bone and fixation with all six screws proved to be unfeasible. The MO models were therefore excluded from the rest of the study. The degree of lateralization of the caudal ilial segment in the LO group was lowest in the TC/LO model and highest in the SC/LO model ([Table 4]).

Table 4 Distance between cranial plate and ilium after rotation

2.5 PO model	TC/LO	SC/LO	BC/LO	TC/MO	SC/MO	BC/MO
Gap plate/ilium (mm)	6.6	9.5	8.7	11.7	14.4	12.6

Total gain of FHC was highest in the TC/LO model (53.7%) and lowest in the BC/LO model (35.5%), similar to the findings in the TPO models. The proportion of FHC gain between the cranial and caudal half of the femoral head was almost even in the TC/LO model. The SC/LO and especially the BC/LO model showed relatively larger FHC gains in the caudal half of the femoral head ([Table 5]).

Table 5 FHC, gain of FHC after rotation, and gain ratio between cranial (cr) and caudal (cd) half of the femoral head

2.5 PO model	Zero	Zero	TC/LOcr	TC/LOcd	SC/LOcr	SC/LOcd	BC/LOcr	BC/LOcd
FHC (mm2)	126	105	182	173	165	167	148	165
Gain FHC (mm2)			56	68	39	62	22	60
Ratio			1:1.2		1:1.5		1:2.7

Abbreviation: FHC, femoral head coverage.

Discussion

The results of this study suggest that in the triple and the 2.5 PO, an ilial osteotomy perpendicular to the pelvic floor (TC) provides greater and more evenly balanced FHC compared with the traditional ilial osteotomy angles. In the 2.5 PO, the TC osteotomy results in a greater bone contact area at the ilial osteotomy site, potentially improving stability and bone healing. At the same time, the TC osteotomy is associated with the least lateral displacement of the caudal ilial segment, thereby reducing bending and shearing forces on bone and implants. The ischial osteotomy in a sagittal plane (LO) appears to be crucial for good bone–implant congruency in the 2.5 PO.

In the TPO group, the results of this study show the smallest change in interischiatic tuberosity distance (3 mm) using a Slocum-type ilial osteotomy. In comparison, the displacement of the ischiatic tuberosity in the BC model (20 mm) is markedly greater, potentially impeding rotation by soft tissue resistance to excessive lateralization. In 2.5 PO a BC-type ilial osteotomy might correspondingly lead to increased shearing forces on the ischial osteotomy site or the ischial table. Similar to the SC model, the TC model showed a relatively small but medial displacement. In the 2.5 PO the tendency for medial displacement could result in a compression force across the ischial table, which might be preferable over a distracting force regarding the probability of postoperative ischial fracture.

The goal of pelvic osteotomy surgery is to increase the weight-bearing surface of the dysplastic hip joint through an increase in FHC to improve joint mechanics and slow the progression of osteoarthritis.[1] [2] Achieving sufficient FHC with a low degree of acetabular rotation would reduce recoil forces of the rotated segment. Therefore, the TC ilial osteotomy might be preferable due to its superior effect on FHC, especially in combination with acetabular rotation of 20 degrees or less, as proposed by Janssens and colleagues.[17]

Moreover, the distribution of FHC gain is also most balanced in the TC model. The more caudally the ilial osteotomy is angled, the more it favors coverage of the caudal half of the femoral head. In the authors' opinion, in the TPO this can be attributed to anteversion of the acetabulum caused by the more caudally angled plane of rotation. Potentially this could have clinical relevance, since the cranial and caudal parts of the acetabulum equally contribute to load bearing in the hip joint.[16]

Bending and shearing forces on implants and bone play a key role in the development of complications like implant failure and bone fracture in pelvic osteotomy procedures. Rotation of the acetabulum ideally should neither lead to incongruency between bone and implants nor to strong elastic recoil forces, while at the same time sufficiently increasing FHC. In the 2.5 PO, the bone overlap at the ilial osteotomy site is largest using a TC-type osteotomy (66.7%) compared with the SC (50%) and BC (37.9%) models. The study also shows the least lateralization of the caudal ilial segment in the TC models. In consequence, reduction of the ilial bone segments to a position congruent with the plate would require fewer bending forces using a TC-type ilial osteotomy.

In the 2.5 PO models with the ischial osteotomy in the sagittal plane (LO), the highest total FHC and the most equal distribution of FHC gain is seen with the TC osteotomy. Similar to the TPO models, although less pronounced, the more caudally angled osteotomies favor coverage of the caudal femoral head. However, the reason for this cannot be the same as in TPO, because in 2.5 PO the axis of rotation does not change with different ilial osteotomy angles. Nevertheless, increased lateralization of the caudal ilium after rotation would require increased inward bending during plate fixation. The resulting change in the orientation of the acetabulum could therefore, in the authors' opinion, account for the difference in FHC distribution between the 2.5 PO models.

A sagittal position of the ischial mono-cortical osteotomy (LO) has been shown to be superior compared with a medially angled osteotomy (MO) regarding bone contact area at the ilial osteotomy site. Reduction and plate fixation in the MO group was not feasible.

The main limitation of this study is the use of a simple, synthetic pelvis as a model for the complex behavior of bone and soft tissue. The questions posed in this study are of geometrical nature and not necessarily dependent on the specific properties of bone and soft tissue. However, in vivo, the elastic components of the bone, the weight load, and the age of the subject can create variables.

The use of a 20-degree TPO plate follows the recommendation of former publications by various authors[18] [19] and also was chosen to avoid excessive FHC, as the models were replications of non-dysplastic hips.

The study did not include double pelvic osteotomy models, because bending properties of the pelvic symphysis and particularly the ischial table could not be sufficiently emulated by the rigid polyethylene model. This is a limitation of the study, as DPO is certainly more common than 2.5 PO and much more widely clinically described. Any effects in DPO can only be extrapolated from the 2.5 PO models but have not been demonstrated and would need further investigation.

It should be remembered that this is an in vitro study, and that the in vivo application of these findings will need to be verified. In particular, it will be necessary to investigate if this ilial osteotomy parallel to the pelvic floor might increase the risk for excessive FHC and subsequent femoroacetabular impingement, even when using plates with small plate angles.

Conclusion

The results of this study suggest that an ilial cut performed perpendicular to the pelvic floor potentially has several advantages compared with more caudally angled ilial osteotomies in both TPO and 2.5 PO surgery.

While achieving the highest gain and most balanced distribution in FHC in the TPO and the 2.5 PO, the TC ilial osteotomy also results in significantly less lateralization of the ilial portion of the acetabular segment after rotation in the 2.5 PO model.

The authors postulate that this type of ilial osteotomy potentially reduces shearing and bending forces on the ilial bone and therefore may reduce stress on the bone–implant interface. It might also be speculated if the TC ilial osteotomy could provide sufficient FHC using smaller angles of acetabular rotation, potentially reducing elastic recoil forces.

The results of the study also strongly support the technique for ischial osteotomy as it has formerly been described for 2.5 PO,[3] since deviation from the sagittal plane potentially results in a high degree of incongruency at the ilial osteotomy site.

Further investigation in clinical trials would be necessary to determine the clinical relevance of our findings.

Conflict of Interest

The authors declare no conflict of interest.

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Publication History

Received: 13 September 2025

Accepted: 09 December 2025

Article published online:
30 December 2025