Augmentation of core decompression with synthetic bone graft does not improve mechanical properties of the proximal femur

Abstract

Core decompression is a minimally invasive surgical technique used to treat patients with avascular necrosis of the femoral head. The procedure requires an entry hole in the lateral cortex of the femur which potentially leaves patients susceptible to subtrochanteric fractures. The purpose of this study was to determine if filling the core decompression tract with synthetic bone-graft mechanically strengthens the proximal femur. Twenty composite synthetic femurs underwent a core decompression procedure; ten were augmented with synthetic bone-graft (PRO-DENSE™, Wright Medical) and ten femurs were left unfilled as a control group. Compressive testing to failure was performed using a mechanical testing machine. Stiffness, fracture load, and toughness did not significantly differ between groups. More subtrochanteric fractures were seen in the control group (6 of 10 specimens) compared to the bone-graft augmented group (2 of 10 specimens). In conclusion, augmentation of a core decompression tract does not improve mechanical properties in a synthetic bone model but may be protective of subtrochanteric fracture.

1. Introduction

Avascular necrosis (AVN) or osteonecrosis of the hip is a common cause of hip pain and subsequent arthritis often leading to the need for total hip replacement (Issa et al., 2013). Insufficient blood supply of the femoral head leads to tissue death and subsequent collapse of the bone under the weightbearing cartilage (Petek et al., 2019). After collapse of the femoral head, total hip replacement is the main treatment option. Prior to collapse, other options are possible that try to improve blood supply and healing to the subchondral bone of the femoral head. These options include medications, core decompression, bone grafting, and free fibular transplantation (LAI et al., 2005; Hua et al., 2019; Han et al., 2020; Cao et al., 2017).

Core decompression is the most common hip preservation procedure and constitutes of drilling a hole into the lesion within the femoral head. The hope is to both increase blood flow to the lesion and to decrease intraosseous pressure which reduces pain (Serong et al., 2020; Wang et al., 2019). Different techniques exist for core decompression from the use of multiple small drill holes to the use of larger holes that can then be packed with bone graft, or even bone marrow cells (Wang et al., 2019). A downside of the larger single hole technique is that the larger drill hole in the lateral cortex may weaken the bone. Subsequent falls or torque may lead to a subtrochanteric fracture, a catastrophic complication requiring either fixation or complex total hip arthroplasty. Subtrochanteric fracture rates through the lateral starting hole in meta-analysis are about 0.9% (Hua et al., 2019).

The objective of this study was to experimentally determine the mechanical effects of using a synthetic bone graft filler for augmentation of a core decompression tract. The bone graft filler is a composite of calcium sulfate and calcium phosphate and hardens rather quickly but is resorbed over time and may act to strengthen the hole in the side of the bone. We hypothesize that a core decompression augmented with synthetic bone graft will have more mechanical strength of the femur during compressive loading than a control and less risk of subtrochanteric fracture.

2. Materials and methods

Twenty composite synthetic femurs (4th Gen, 17 PCF, Medium, Sawbones, Pacific Research Labs, Vashon Island, WA; synthetic cortical bone compressive strength of 157 MPa; solid foam core compressive strength of 6.2 MPa) were used. Each had a predrilled hole to the center of the femoral head from the lateral cortex. A board-certified orthopaedic hip surgeon (CLB) placed a guidewire in the predrilled hole and used a 9 mm cannulated drill to make a core decompression tract of 83 mm in length. Ten femurs were then filled with synthetic bone graft (5.3 cc, PRO-DENSE™, Wright Medical, compressive strength of 40 MPa) using the company syringe and injection device and were allowed to set for 24 h. Uniform filling of the drilled cavities was confirmed via radiography. The remaining ten femurs were left unfilled and served as the control group. The bottom 6 cm of each specimen was potted using auto-body filler (Bondo®, 3 M, St. Paul, MN) in to a 5 cm diameter cylindrical aluminum pots (Palumbo et al., 2014). For testing, the femur diaphysis was fixed at a 20° angle from vertical to model anatomical position (Sikes et al., 2011; van der Steenhoven et al., 2009). The aluminum pots were secured to an MTS 858 Bionix® Test System (MTS Systems Crop, Eden Prairie, MN) for compressive load testing. A 90° bracket was secured to the actuating arm to simulate supporting tissues and to prevent varus torque during compressive testing (Fetto, 2019) (Fig. 1).

Fig. 1.

Mechanical testing setup.

TestWorks® 4 software (WR Medical Electronics Co, Maplewood, MN) was used to program compression-to-failure testing with a 10 N preload and 2 mm/s displacement rate (Sikes et al., 2011; van der Steenhoven et al., 2009; Fetto, 2019). Load and displacement data (500 Hz) from the test system were analyzed using custom code (MATLAB, Mathworks, Natick, MA). Stiffness was calculated as the slope of the linear elastic region of the load-deformation curve. Fracture load values were taken as the maximum recorded force during testing. Toughness (representing the amount of energy needed to deform to failure) was calculated as the area under the load-deformation curve. Two-sample t-tests were performed to analyze significant differences between the augmented and control synthetic femurs (p = 0.05). Observations of fracture patterns were also recorded using the Orthopaedic Trauma Association (OTA) 2018 classification system by a board-certified orthopaedic surgeon (SCM) (Meinberg et al., 2018).

3. Results

Compressive failure testing for control (n = 10) and augmented (n = 10) synthetic femurs showed no significant differences between groups in stiffness, fracture load, or toughness (Table 1).

Table 1.

Mean (standard deviation) of mechanical testing parameters for augmented and control synthetic femurs, with p-values. No values were statistically different between groups.

	Stiffness (N/mm)	Fracture Load (N)	Toughness (J)
Control	2151 (195)	9074 (525)	20.1 (3.0)
Augmented	2195 (169)	9374 (1223)	21.3 (3.2)
p-value	0.598	0.485	0.425

Mean stiffness values for augmented synthetic femurs were 2.0% greater, mean fracture load was 3.3% greater, and mean toughness was 5.6% greater compared to control specimens, though none reached statistical significance. There was, however, an observable difference in fracture pattern between the two groups (Fig. 2D). When tested to failure, most of the control specimens sustained subtrochanteric fractures (OTA 31A3.2, Fig. 2C), while most of the augmented specimens sustained femoral neck fractures in a basicervical pattern (OTA 31B3, Fig. 2B). One control and two augmented specimens sustained femoral head fractures (OTA 31C1.3 Fig. 2A). All of the subtrochanteric patterns for both groups fractured at the core decompression site on the lateral subtrochanteric surface.

Fig. 2.

Examples of the fracture patterns observed during testing: (A) femoral head OTA 31C1.3, (B) femoral neck OTA 31B3, (C) subtrochanteric OTA 31A3.2 fracture patterns, and (D) bar graph of occurrence of fracture types in each group.

4. Discussion

The goal of this study was to determine if augmentation of a core decompression tract with bone graft substitute would increase the mechanical properties of a synthetic bone and prevent subtrochanteric fracture. Our study showed no improvement in mechanical properties of the bone when compared to control specimens. Previous work has shown that the size of the core decompression tract is related to mechanical properties. Using finite element modelling, a tract diameter of 8 mm was shown to weaken the bone when compared to smaller tract sizes (Tran et al., 2014). The position of the tract is also important as a more distal starting point leads to high risk of subtrochanteric fracture in finite element modelling (Cilla et al., 2017). Our study utilized a 9 mm tract diameter with a consistent starting point and showed no differences in mechanical properties due to augmentation. These results are consistent with a previous finite element study that shows augmentation does not provoke additional risk of subtrochanteric fracture (Tran et al., 2014). Kok et al. utilized a novel technique of patient-specific computational modelling pre- and post-core decompression with augmentation, then computationally modified variables such as bone graft volume, injection location, and stiffness of the material (Kok et al., 2019). Consistent with the insignificant slight increases in mechanical strength of 2–6% in our study, their results indicate increases of 0–9% in strength from the clinical post-operation models, but they found an increased value of the bone graft when variables were computationally modified. The authors discuss their results in the context of clinical relevance: how much strength increase is clinically meaningful for patients? The results of our experimental study suggest that in addition to strength, a change fracture location due to augmentation may be of interest for future studies.

Clinically, the approach of using synthetic bone grafting has provided favorable outcomes. In a review of 31 patients treated with this technique, 76% were successful and no subtrochanteric fractures were seen (Landgraeber et al., 2017). In our study, while strength increases were not significant, there appear to be differences in fracture patterns between groups. The augmented synthetic femurs tended to break at the femoral neck while the control group more often fractured at the subtrochanteric level, suggesting that augmentation may decrease the risk of subtrochanteric fracture. According to a clinical meta-analysis, the risk of subtrochanteric fracture is about 0.9% (13 out of 1440 in 21 combined studies) (Hua et al., 2019). Although augmentation may offer some benefit, the risk of fracture may have more to do with location of the tract, size of the hole, and the use of extra drill holes. While our study on synthetic femurs showed fewer subtrochanteric fractures in the augmented group, the benefits of bone grafting may be more related to healing of the tissue and potential for ingrowth and remodeling (Serong et al., 2020), factors we were unable to model in the current study. Furthermore, the additional surgical time and cost when using bone grafting techniques are considerations the surgeon must balance with a possible risk of decreased subtrochanteric fractures.

Previous studies have examined the use of polymethylmethacrylate cement (PMMA) to augment the proximal femur and prevent potential osteoporotic fracture. While PMMA cement is stiffer than synthetic bone graft material, this study may give insight into how much material is needed to mechanically strengthen the proximal femur. In previous studies, when the proximal femur was filled with cement (40–50 cc), mechanical strength was increased (Sutter et al., 2010a). With smaller more selective amounts (15 cc, closer to the amount in present study), strengthening was not apparent (Sutter et al., 2010b). Similarly, when 15 cc of cement was placed in a V-shaped pattern, mechanical improvements were not observed (Raas et al., 2016). However, there is also concern that larger amounts of cement, that subtrochanteric fracture patterns could be potentiated (Sutter et al., 2010a).

This study was not without limitations. The synthetic femurs mimic the biphasic nature of human bones and the morphology is based on that of a natural femur, but biomimetic materials are not identical to living biological material, particularly when considering in vivo wet conditions This study utilized a single loading-to-failure experimental design, whereas in vivo forces felt by the femur are not as simple. The impact of cyclic loading or complex loads on core decompression techniques remains unknown. Similarly, the surrounding tissues such as the acetabulum, hip joint capsule, and skeletal muscles greatly affect the forces felt by the femur in vivo and are not included in our model. The synthetic femurs in this study utilize a homogenous foam core to model the cancellous bone which does not allow for spreading of the graft material past the drilled tract. Additionally, various types of bone grafts exist, and the results of this study may not extend to all graft types. While the sample size of this study (n = 20) was likely sufficient to show differences in the mechanical testing results, a larger sample size may elucidate statistical differences in fracture patterns.

5. Conclusion

Synthetic bone graft is sometimes used to augment a core decompression in patients with avascular necrosis, yet no added mechanical strength from the filler was found. However, bone graft may relocate stresses to prevent subtrochanteric fracture at the entry site.

Abbreviations:

AVN

avascular necrosis

OTA

Orthopaedic Trauma Association

THA

total hip arthroplasty