Exploring the optimal reconstruction strategy for Enneking III defects in pelvis bone tumors: a finite element analysis
Managing pelvis tumors presents a formidable challenge for orthopedic oncologists. Several pelvis reconstruction methods, such as allografts, autografts , hip transposition , and prosthetic reconstruction , are available. Prosthetic reconstruction has recently gained popularity because of its high stability, relatively few complications, aesthetic benefits, and early weight-bearing capacity. However, controversy exists reconstruction for Enneking III defects in the pelvis . To maintain weight-bearing continuity and avoid complications such as fractures and nonunion, region III reconstruction is typically avoided . However, complications can arise from not reconstructing region III, with postoperative hernias being the most common . Therefore, some studies advocate that bone reconstruction enhances functionality and reduces complications, supporting the use of mesh and soft tissue fixation . However, research reports a higher infection rate, prosthesis fractures, and an increased risk of pubic symphysis deformation after region III bone reconstruction . Overall, further research is needed to determine whether bone reconstruction for region III defects is advisable. We were unable to identify studies that have used the FEA method to explore the utility of reconstructing the pelvis Enneking III region. FEA divides a complex structure into finite elements and elucidates mechanical behavior using element-based calculations. FEA is widely used in mechanical stress and failure analysis and has frequently been applied to the pelvic region . Therefore, FEA is a powerful tool for exploring the biomechanical characteristics of pelvis prosthetic reconstruction. We used the FEA method to examine the stress and deformation levels of three Enneking III reconstruction schemes—the Enneking I + II + III, II + III, and isolated III regions—during normal walking. The purposes were to clarify (1) the utility of reconstructing the pelvis Enneking III region and (2) the optimal approach for this reconstruction. 

We obtained three CT datasets of patients with pelvis tumors from our hospital's image database and constructed FEA models for three types of pelvis bone defects. Type 1 represents Enneking I + II + III, involving defects of the ilium, acetabulum, ischium, and pubis. Type 2 corresponds to Enneking II + III, with defects of the acetabulum, ischium, and pubis. Type 3 represents Enneking III, involving defects exclusively in the ischium and pubis. For Types 1 and 2 defects, we initially designed prosthetic reconstruction schemes excluding region III defects, whereas Type 3 defects remained unreconstructed. We then followed the method of Iqbal et al. , replacing all region III areas with a sizable solid box and performing a topology optimization (TO) to identify the optimal reconstruction method for region III (Fig. A–C). Based on the defect location, TO results, and our clinical experience, we developed three reconstruction options for each type (Fig. D–L). Scheme A involves no reconstruction of region III. Scheme B is a relatively simple reconstruction, involving fewer or no contralateral pelvis regions. Scheme C is a more complex reconstruction, involving more contralateral pelvis regions. Following a previously reported protocol , three-dimensional (3D) pelvis models were constructed on the basis of DICOM-formatted CT data using Mimics software (version 21.0, Materialise, Leuven, Belgium). The pelvis-prosthesis FEA models were then built using Geomagic Wrap software (version 2017, USA), SOLIDWORKS software (version 2021, Dassault Systemes, France), and HyperMesh software (version 2022, Altair Engineering Inc, USA). A mesh sensitivity analysis of the pelvis FEA model revealed that a mesh size less than 1 mm limits the impact on the FEA results to less than 5%. Therefore, the mesh size of the pelvis-prosthesis model was set uniformly to 1 mm. Next, we will assign material properties to the pelvis. Previous studies have shown that bone material properties can be categorized as homogeneous or inhomogeneous, with the former reducing FEA complexity but potentially yielding less accurate results . Therefore, we chose inhomogeneous material properties, which are more complex but improve FEA accuracy . Inhomogeneous bone material properties are assigned in the Mimics software on the basis of the corresponding gray-value using Eqs.  and  . Finally, the model was imported into ANSYS software (version 2022R1, Canonsburg, Pennsylvania, USA), where other material properties were assigned, and FEA and TO were conducted. The material properties of each part of the model are detailed in Table . The load magnitude and direction presented in Table simulate the most common of the six main human activities: pelvis loading during normal walking (data referenced from Iqbal et al. ). A full constraint was applied to the superior surface of the sacrum. All contacts in the FEA model were defined as bonded to simulate the force conditions following osteointegration of the prosthesis and host bone, which typically occurs at least 6 months postoperatively . The biomechanical characteristics of each reconstruction scheme were evaluated by assessing the peak stress, stress distribution, and deformation of the pelvis, prosthesis, screws (excluding region III), and region III. 1 [12pt]{minimal}
				
				 
				 
				 
				
				
				
				
				$$ =6.9141{e}^{-4} HU+1.026716$$ ρ = 6.9141 e - 4 × H U + 1.026716 2 [12pt]{minimal}
				
				 
				 
				 
				
				
				
				
				$$E=2017.3 { }^{2.46}$$ E = 2017.3 × ρ 2.46 

TO results are presented in Fig. . For Types 1 and 2 defects, the TO results are hollow, making reconstruction challenging; thus, we developed reconstruction schemes on the basis of experience. In Scheme B, one screw is used to anchor the prosthesis to the contralateral pubis. In Scheme C, the screws in Scheme B are replaced with a pubic plate, and one contralateral anterior acetabular column screw and one contralateral pubic screw are added. However, for Type 3 defects, the TO result resembles the original shape of the ischium and pubis, so we designed reconstruction prostheses on the basis of the original shape. Additionally, Schemes B and C both include two anterior acetabular column screws and two ischium screws, with Scheme C further adding one contralateral anterior acetabular column screw and one contralateral pubis screw. Type 1 defects Figure and Table present the FEA results for Type 1 defects. Scheme C demonstrates a more uniform stress distribution for the pelvis with a significant reduction in peak stress compared with both Schemes A and B (Fig. A–C). However, pelvis deformation increases in Scheme C (Fig. D–F). For the prosthesis (Fig. G–L), Scheme C results in the lowest stress and deformation levels. With respect to the screws (excluding region III) (Fig. M–R), Scheme C reduces both the stress and deformation compared with those of Schemes A and B. In Region III (Fig. S, ), Scheme C has a higher stress but significantly lower deformation compared to Scheme B (Fig. U, ). Type 2 defects Figure and Table present the FEA results for Type 2. Compared with both Schemes A and B, Scheme C results in a reduction in the peak stress of the pelvis (Fig. A–C); however, the degree of pelvis deformation is slightly greater than that in Scheme B (Fig. D–F). For the prosthesis (Fig. G–L), Scheme C results in the lowest stress and deformation values. In terms of screws (excluding region III) (Fig. M–R), Scheme C achieves notable stress reduction compared with Schemes A and B, with minimal differences in deformation. In region III (Fig. S, ), Scheme C results in significantly greater stress and deformation compared to Scheme B (Fig. U, ). Type 3 defects Figure and Table show the FEA results for Type 3 defects. Schemes B and C show lower stress levels near the affected sacroiliac joint compared to Scheme A, with Scheme C achieving the most uniform stress distribution and the lowest deformation (Fig. A–F). For the prosthesis (Fig. G–J), although Scheme C results in a higher peak stress than Scheme B does, it results in a more uniform stress distribution and reduced deformation. With respect to the screws (Fig. K–N), Scheme C reduces both the stress and deformation compared with Scheme B. 

Figure and Table present the FEA results for Type 1 defects. Scheme C demonstrates a more uniform stress distribution for the pelvis with a significant reduction in peak stress compared with both Schemes A and B (Fig. A–C). However, pelvis deformation increases in Scheme C (Fig. D–F). For the prosthesis (Fig. G–L), Scheme C results in the lowest stress and deformation levels. With respect to the screws (excluding region III) (Fig. M–R), Scheme C reduces both the stress and deformation compared with those of Schemes A and B. In Region III (Fig. S, ), Scheme C has a higher stress but significantly lower deformation compared to Scheme B (Fig. U, ). 

Figure and Table present the FEA results for Type 2. Compared with both Schemes A and B, Scheme C results in a reduction in the peak stress of the pelvis (Fig. A–C); however, the degree of pelvis deformation is slightly greater than that in Scheme B (Fig. D–F). For the prosthesis (Fig. G–L), Scheme C results in the lowest stress and deformation values. In terms of screws (excluding region III) (Fig. M–R), Scheme C achieves notable stress reduction compared with Schemes A and B, with minimal differences in deformation. In region III (Fig. S, ), Scheme C results in significantly greater stress and deformation compared to Scheme B (Fig. U, ). 

Figure and Table show the FEA results for Type 3 defects. Schemes B and C show lower stress levels near the affected sacroiliac joint compared to Scheme A, with Scheme C achieving the most uniform stress distribution and the lowest deformation (Fig. A–F). For the prosthesis (Fig. G–J), although Scheme C results in a higher peak stress than Scheme B does, it results in a more uniform stress distribution and reduced deformation. With respect to the screws (Fig. K–N), Scheme C reduces both the stress and deformation compared with Scheme B. 

This study used FEA and TO methods to develop reconstruction schemes for three types of Enneking III defects in the pelvis. For each type of defect, three reconstruction schemes were proposed: non-reconstruction (Scheme A), simple reconstruction (Scheme B), and complex reconstruction (Scheme C). This study aimed to biomechanically assess the utility of reconstructing Enneking III defects in bone tumors and identify the optimal reconstruction strategy. The FEA results show that both Schemes B and C generally outperform Scheme A biomechanically, achieving a more uniform stress distribution and lower peak stresses in the pelvis, as well as reduced peak stresses and deformations in the prostheses and screws, with complex reconstruction performing better than simple reconstruction. This reduces the risk of stress fractures and prosthesis fractures. Reducing peak stress and achieving uniform stress distribution are crucial for reconstructing pelvis bone defects with prosthetics. This helps prevent pathological fractures and breakage of prostheses or screws, thereby enhancing the long-term stability of the prosthesis. The FEA results show that the pelvis stress is concentrated at the fixed edge of S1 on the side with greater force. In Types 1 and 2 pelvis defects involving the acetabulum, stress levels in the pelvis, prosthesis, and screws (excluding region III) are lower in both reconstruction schemes than in the Scheme A, with Scheme C outperforming Scheme B. Additionally, stress in the defect area (region III) is concentrated at the connection with the artificial acetabulum. For Type 3 defects, which involve only a defect in the ischium and pubis, Scheme C is lower than Scheme B in terms of the prosthesis peak stress but higher in terms of the peak stress of the screw. However, the stress levels in the pelvis bone are similar for both reconstruction schemes. Most importantly, for all three types, the peak stress in the pelvis bone under Scheme C is lower than the yield strength of the cortical bone (150 MPa) . Additionally, the peak stress in the prosthesis and screws is significantly lower than the yield strength (789–1013 MPa) and fatigue limit (310–610 MPa) of Ti–6Al–4V . As a result, reconstructing region III reduces the risk of complications such as prosthesis and screw loosening and breakage, with complex reconstruction outperforming simple reconstruction. Deformation of the pelvis and prosthesis is a key indicator for assessing reconstruction stability. Smaller deformation indicates less local stress concentration and improved reconstruction stability . For Types 1 and 2 defects, first, the maximum total deformation of the pelvis is near the intact obturator foramen, and the maximum total deformation of the prosthesis is at the edge of the artificial acetabulum. Second, for total pelvis deformation, both Schemes B and C for Type 1 defects result in greater deformations than the Scheme A does, whereas the opposite is true for Type 2 defects. Third, for the prosthesis and screw deformation levels (excluding region III), both Schemes B and C for Types 1 and 2 defects are lower than the Scheme A. Furthermore, for Type 1 defects, Scheme C outperforms Scheme B. However, for Type 2 defects, the two reconstruction schemes are similar. For Type 3 defects, the maximum total deformation in the pelvis is near the affected side obturator foramen. Moreover, in terms of total deformation of the pelvis, prosthesis, and screws, both Schemes B and C are lower than in the Scheme A scenario, with Scheme C outperforming Scheme B. Overall, reconstructing region III effectively enhances the stability of bone defect reconstruction compared with non-reconstruction. The pelvis is a closed ring structure formed by the sacroiliac joints and the pubic symphysis. The anterior ring, which includes the pubic symphysis, contributes 40% of the overall stability of the pelvis ring . Moreover, loss of the pubic symphysis increases the load on the sacroiliac joints, leading to joint instability and osteoarthritis . As a result, the integrity of the anterior ring is crucial for stress distribution in the pelvis. Growing research supports the construction of a complete pelvis ring . For example, Wang et al.  performed hemipelvic prosthesis reconstruction on 11 patients with periacetabular bone tumors, with an average follow-up time of 15.5 months. The results revealed an average MSTS-93 lower limb function score of 19.2. Wang et al.  designed pelvis ring reconstruction prostheses for 13 patients with periacetabular bone tumors and conducted short-term follow-up. The results revealed a median MSTS-93 score of 23, a median VAS score of 2, and successful bone integration for all prostheses. Zhang et al.  conducted an average 23.6-month follow-up for 5 patients who underwent Type 3 hemipelvic prosthesis reconstruction and reported an average MSTS score of 29.8, with bone integration observed in all patients. Jamshidi et al.  conducted a mean 6-year follow-up on 32 patients who underwent Type 3 hemipelvic resection (15 reconstructed, 17 unreconstructed). The results revealed significantly higher average MSTS scores in the reconstructed group than in the unreconstructed group (26 and 22.7, P < 0.001), with lower average VAS scores postoperatively (2.1 and 4.2, P = 0.016). These studies confirm that bone reconstruction in region III results in improved function and fewer complications. Moreover, reconstruction of region III also supports soft tissue reconstruction and can enhance the biomechanical stability of pelvis reconstruction by evenly distributing deformation and stress through the addition of pedicle screw systems . However, some studies disagree with the reconstruction of defects in region III. First, Dong et al.  reported found that bone reconstruction, including that in region III, for Type I + IV bone defects results in significant vertical deformation at the pubic symphysis. Second, for Type I + II + III or II + III bone defects involving the acetabulum, reconstruction via a pubic plate or other methods may lead to stress concentration on the pubic plate, increasing the risk of fracture. Reconstruction also prolongs the surgical time, increasing the chances of infection and bleeding . Third, according to our clinical experience, the presence of important blood vessels and nerves in region III makes these structures susceptible to injury during reconstruction, leading to complications such as bleeding, delayed wound healing, or sensory abnormalities. Therefore, significant controversy remains regarding the utility of reconstructing region III. Encouragingly, this study revealed that reconstructing region III resulted in a more uniform distribution of pelvis stress and a more stable prosthesis from a biomechanical standpoint. This sheds light on the question of whether to reconstruct region III. This study has several limitations. First, we applied only one type of load (normal walking) and did not assess outcomes under different daily activities and gait cycle loads. Although normal walking was used as a standard reference, future studies should include patient-specific conditions, such as fast gait and limping gait, to better reflect postoperative variability . Second, to streamline calculations and obtain results efficiently, our FEA model simplifies the screws to cylinders. This simplification may influence the accuracy of stress and deformation results, particularly in regions where the interaction between screws and surrounding bone is critical. Additionally, the exclusion of muscles and ligaments may explain why the maximum deformation in the pelvic model occurs at the bottom of the ischium, rather than at the iliac wing, which is where maximum deformation is observed in FEA studies that include these structures . Third, this study assumed full bonding between the host bone and the implant to explore optimal conditions; however, future studies should simulate partial bonding scenarios to better reflect real-world conditions. Fourth, this study relied exclusively on numerical research through FEA and omitted mechanical compression and clinical experiments to validate the findings. Finally, a limitation of FEA is that it simplifies the model, often excluding key structures such as blood vessels and nerves. As a result, prosthesis deformation during FEA may conflict with these sensitive structures, potentially leading to complications such as necrosis or amputation in clinical applications. However, these anatomical structures can be incorporated into the prosthesis design and FEA in future research to achieve a more balanced outcome. 

The FEA results suggest that reconstructing Enneking Zone III defects, regardless of defect type, results in a more uniform stress distribution and lower peak stress in the pelvis compared to non-reconstruction under simulated walking loads. Stress levels remain below the yield strength of cortical bone, while prostheses and screws experience reduced peak stress and deformation, potentially increasing biomechanical stability and reducing the risk of fractures. Complex reconstruction schemes involving more contralateral pelvis regions demonstrate superior biomechanical performance compared to simpler schemes. However, biomechanical advantages observed in FEA simulations do not fully account for real-world clinical complexities when performing reconstruction, such as prolonged operative times, vascular and nerve injuries, or risks of reconstruction failure. These factors, along with patient preferences and oncologic prognosis, must be carefully considered. Finally, while reconstruction may offer biomechanical benefits, clinical decisions should be individualized, balancing biomechanical insights with patient-specific factors. 

