Authors Harry Hothi, Sean Bergiers, Johann Henckel, Anna Di Laura, John Skinner, Alister Hart, of the Royal National Orthopaedic Hospital have studied orthopaedic implants from 29 countries
Joshua Jacobs, MD, former president of the American Academy of Orthopaedic Surgeons (AAOS) is quoted as saying “It is hard to overstate the importance of implant retrieval analysis in the development of successful orthopaedic devices” . The reason is clear: by understanding how implants perform today, we can build longer lasting implants in the future.
Analysis of implants retrieved from the body can give us many clues about their performance; when this engineering data is combined with clinical and medical imaging data, we’re able to better understand the surgeon, implant and patient factors which may influence this performance.
At the Royal National Orthopaedic Hospital (RNOH) Implant Science Centre, we’ve collected over 10,000 spine, hip and knee implants from patients in 29 different countries. Many of these have been manufactured using traditional cast or wrought methods and we’ve developed metrology methods to assess how these have worn and corroded while in use. This has involved the use of optical and contact profilometry and coordinate measurement machines to characterise surfaces changes and damage.
Orthopaedic implant engineering has recently experienced a revolution with a shift towards more implants being produced through additive manufacturing (AM) methods (also known as 3D printing).
The benefits of AM include a greater control over the production of highly porous surfaces that can better adhere to bone, and the design of customised implants to fit patients with complex bony anatomies.
Over 300,000 patients receive an orthopaedic implant (knee, hip, spine) every year in the UK alone, at a cost of over £2.5Bn; 15% of revision hips are now AM implants, a 3-fold increase over the past decade.
However, all new methods have risks, which are high for AM orthopaedic implants particularly because over a short period of time there has been rapid adoption of this technology. In addition to being able to characterise the surfaces of these implants, we must now also be able to look inside them to better understand their microstructural properties.
Micro-computed tomography (Micro-CT) is now being utilised to comprehensively assess the porous and internal structures of these AM devices. In contrast to traditional CT imaging, its greater resolution allows the characterisation of potentially performance-influencing, micron-scale features.
Such investigations have highlighted the variability in current designs of porous titanium implants, with respect to their morphology, regularity, porosity, pour size and strut thickness [2-4]. Although these structures were introduced to promote bone ingrowth and enhance fixation, there is currently no consensus as to which design provides the best environment to achieve is common goal.
Internal cavities have recently been identified within the dense walls of AM orthopaedic implants, through the adoption of micro-CT imaging . Thus far, there is no evidence to suggest that these imperfections detrimentally effect the structural integrity of these devices; nevertheless, as they are a direct consequence of the AM process, they should be closely monitored to better understand their clinical significance.
The porous surfaces of AM implants should theoretically enhance bone ingrowth and therefore their fixation in patients. Our recently published study investigating retrieved AM cups used in total hip replacement surgery, used histological analysis to show that these AM cups had greater bone ingrowth than similar cups that were conventionally manufactured .
To conclude, AM implants have demonstrated positive clinical outcomes to date, however there are several known limitations of the technology that we must be aware of as new designs are developed.
 Jacobs JJ, Wimmer MA. 2013. An important contribution to our understanding of the performance of the current generation of metal-on-metal hip replacements. The Journal of Bone & Joint Surgery, 95-A, 8, e53(1-2).
 Dall’Ava L, Hothi H, Henckel J, Di Laura A, Shearing P, Hart A. 2019. Comparative analysis of current 3D printed acetabular titanium implants. 3D Printing in Medicine, 5, 15: 1-10.
 Dall’Ava L, Hothi H, Henckel J, Di Laura A, Bergiers S, Shearing P, Hart A. 2020. Dimensional analysis of 3D-printed acetabular cups for hip arthroplasty using X-ray microcomputed tomography. Rapid Prototyping Journal, 26/3: 567-576.
 Dall’Ava L, Hothi H, Henckel J, Di Laura A, Shearing P, Hart A. 2020. Characterisation of dimensional, morphological and morphometric features of retrieved 3D-printed acetabular cups for hip arthroplasty. Journal of Orthopaedic Surgery and Research, 15 (157): 1-12.
 Hothi H, Dall’Ava L, Henckel J, Di Laura A, Iacoviello F, Shearing P, Hart A. 2019. Evidence of structural cavities in 3D printed acetabular cups for total hip arthroplasty. Journal of Biomedical Materials Research Part B: Applied Biomaterials, 108(5): 1-11.
 Dall’Ava L, Hothi H, Henckel J, Di Laura A, Tirabosco R, Eskelinen A, Skinner J, Hart A. 2021. Osseointegration of retrieved 3D-printed, off-the-shelf acetabular implants. Bone Joint Res; 10(7): 388-400.