With the operation technology becoming more and more mature, surgeons and engineers are paying more and more attention to prosthesis designs and modifications with better biomechanics properties to improve THA clinical outcomes21,22. The absence of locking mechanism of bone cement may lead to high early aseptic loosening, so cementless acetabular and femoral components has become more and more popular in THA surgeries23. Multiples studies report short to mid-term clinical outcomes with a large variety of cementless acetabular cups24–26. As is known to all, both initial mechanical stability and long-term biological fixation are essential parts to achieve durable survivorship of acetabular cups. Thus, the biological coating technology becomes a highlight issue. Traditional coated surfaces of the acetabular cup include cobalt chromium molybdenum sintered beads, diffusion bonded fibre metal mesh, cancellous structured titanium and titanium-sprayed plasma, demonstrated a certain osseointegration ability. However, the limitations of these techniques may occur in case of poor host bone qualities and conditions, such as osteonecrosis, osteoporosis, dysplasia and massive bone defect.
The manufacturing process of 3D printing acetabular cup is completely different from that of traditional cup. In traditional reduction casting process, the interface between the solid layer and the coated surface of the acetabular cups may cause detachment and corrosion, resulting to cup failures. But 3D printing, via additive manufacturing process, has made it easier to individualize product design and manufacturing27. Thus, the 3D ACT EBM cup, with more appropriate porosity, ideal porous diameter, lower modulus of elasticity and better coefficient of friction, may provide better solutions for various kinds of primary THAs.
It has been formerly confirmed that EBM manufactured implants, with interconnected porosity and rough surface, could provide a viable medium for bone ongrowth and ingrowth by creating a stable porous construct while also maintaining the mechanical strength in vitro and animal studies13–15. Perticarini12 et al first reported the successful utilization of Trabecular Titanium acetabular cups in European patients, among which are mainly primary hip OA, as well as AVN and DDH patients, and significant functional recovery and pain relief were recorded in all 133 cases at minimum 5 years follow-up.
All cups obtained initial stabilities and long-term fixation in our study. No cup loosening determined according to the DeLee and Charnley’s definition of three zones and excellent bone ingrowth defined by the Anderson Orthopedic Research Institute in our study. Early-stage radiolucent lines within 2 mm showed in 15 cases gradually disappeared at 3 to 6 months postoperatively, which represented efficient bone ingrowth to the porous metal structure. The interface micromotion can be reduced if the coefficient of friction get increased, which can enhance primary fixation and contribute to secondary fixation28,29. Great bone integration property is considered as the EBM-produced cups’ peculiar advantage related to: ①its extremely more rough surface and higher coefficient of friction on cancellous bone of 1.08, compared with that in traditional sintered beads (0.5); and ②its porous, solid and special interconnected structure mimic the trabecular morphology of the natural bone. These properties do have the advantages of osteoinduction and osteoconduction by stimulate vascularisation, osteoblast proliferation and differentiation in previous study30.
It should be mentioned that in China, the biggest developing country in the world, we often encounter severe type of end-stage hip diseases, such as Crowe IV DDH and acetabular protrusion with great bone defect, due to China’s national economical and healthcare conditions. In our study, we are glad to see encouraging great clinical and radiographic outcomes in difficult cases such as Crowe IV DDH, acetabular protrusion with large bone defect and RA patients with severe osteoporosis. Remarkable HHS and WOAMC score improvements and high satisfaction rate ensured the cup’s efficiency among Chinese patients. No acetabular cup failures occurred in any cases, with 100% survival rate till the last follow up, together showed an excellent short to mid-term outcomes of the 3D ACT cups.
This study does have several limitations. First, in this retrospective study, 11/103 of the patients lost to follow up. Second, no controlled groups enrolled in this study, and we are about to carry out a prospective randomized controlled trial for higher-level evidence. Third, cases from 6 different surgeons enrolled which may confound the results. Fourth, computer tomography (CT) scans and bone densitometry evaluations as well as relevant laboratory examinations were not conducted.