Advancing custom implant design: A bone-preserving, patient-specific approach

The Royal National Orthopaedic Hospital is a world leader in researching and managing complex hip joint problems, including those involving significant bone loss in the pelvis (acetabular defects). The hospital is renowned for its expertise in using personalised implants to address these challenges. In this article, authors Anna Di Laura, Johann Henckel, Alister Hart, of the Royal National Orthopaedic Hospital NHS Trust, Stanmore, UK, discuss the current advances in custom implant designs.

Understanding bone loss around hip replacements

One major issue in hip replacements is acetabular osteolysis, a condition where bone loss around the implant weakens its attachment and leads to loosening. This complication significantly affects the long-term success of hip replacement surgery. A better understanding of the factors associated with the progression of osteolysis around implants has been limited by a lack of sensitivity of plain radiography[1].

There are three main sources/causes of significant bone loss around acetabular hip implants.

Metal-on-Polyethylene (MoP) bearings: 

These materials were widely used in hip replacements but were found to wear over time, producing debris that triggers osteolysis.

Metal-on-Metal (MoM) bearings: Introduced as a solution to MoP wear, this generation of implants was expected to reduce bone loss. However, research has shown that metal debris from these implants can also cause osteolysis, sometimes more aggressively than in MoP implants. In some patients the osteolysis from MoM is severe within 10 years of the primary operation.

Peri-prosthetic infection: Infections can cause bone loss by releasing bacterial toxins and triggering inflammation. This inflammation activates cells that degrade bone. Bacteria can also form a biofilm on the implant, which allows the infection to persist and continue harming the surrounding bone. The infection must be treated before definitive reconstruction.

Challenges in detecting bone loss
Diagnosing bone loss early is critical, but standard X-rays are not very effective at detecting osteolysis[1]. This limitation means that the severity of the bone loss may be underreported [2, 3].

• MoP implant osteolysis: Easier to detect on X-rays since polyethylene is X-ray transparent, causing bone loss to show as a dark area.
• MoM implant osteolysis: More challenging to detect, as metal debris deposits appear as “bubble” or “cloud” signs rather than clear bone loss. Blood tests measuring cobalt and chromium ion levels can be useful, as elevated levels may indicate implant wear [4].
• Infection – Even in cases of significant bone loss, X-rays may be limited in detecting peri-prosthetic infections due to poor contrast around the implant and overlapping metal artefacts.

Figure 2: Coronal CT view showing osteolytic lesions around a failed MoM hip implant (red arrows).

The role of advanced imaging

Since X-rays often miss early osteolysis, CT scans are the preferred imaging method for assessing bone loss around hip implants [5, 6]. Although metal implants create imaging artefacts, modern CT software techniques enable better visualisation and measurement of osteolytic lesions.

Surgical treatments

There isn’t a single best treatment for bone loss around hip implants; the most appropriate approach depends on the severity of the bone loss and patient-specific factors.

• Bone grafting: This method has been used to restore bone stock, but its effectiveness varies. While some cases show successful integration, long-term outcomes remain uncertain, particularly in larger defects or in patients with poor bone quality.
• Implant choices: The selection of implants is tailored to the extent of bone loss. Surgeons may opt for hemispherical cups in cases of mild to moderate bone loss, trabecular metal cups with augments for structural reinforcement in more significant defects, or custom 3D-printed implants designed for severe bone deficiencies where standard implants may not provide sufficient stability.

Ultimately, the decision is made based on imaging, bone quality, and the patient’s overall condition to optimise implant fixation and longevity.

Advancements in 3D-printed custom implants

At the Royal National Orthopaedic Hospital, surgeons frequently revise failing MoM and MoP hip implants, treating patients from the UK and abroad. They are increasingly observing severe osteolysis within just 10 years of MoM implant placement, emphasising the need for early diagnosis and intervention.

With extensive experience in using 3D-printed custom implants to reconstruct severe bone defects7-10, the hospital’s clinical and engineering teams, along with industry partners, have pioneered the development of patient-specific titanium implants to restore lost bone structures.

Figure 3: From CT scan to implant design and post-operative radiological outcome workflow. https://register.gotowebinar.com/register/5775681147322486623

Advancing custom implant design: A bone-preserving, patient-specific approach

While the conventional approach to designing custom implants typically requires additional bone removal—often problematic in cases with already compromised bone stock11—the Academic Clinical Orthopaedic Unit at the Royal National Orthopaedic Hospital (RNOH) has partnered with Synopsys Inc., a leading image processing software provider, and implantcast GmbH, a leading medical technology company specialised in custom-made implant manufacturing, to develop a novel, bone-preserving approach to implant design. This strategy offers several key innovations.

• Minimal bone removal
  Traditional implant designs frequently necessitate aggressive bone resection to create adequate space for implantation, potentially weakening already compromised skeletal structures. In contrast, the new approach employs advanced image processing and 3D modelling to create custom implants that conform closely to the patient’s existing anatomy. This patient-specific fit significantly reduces the need for bone removal, helping to preserve native bone stock and improve long-term outcomes.
• Precision placement
  The custom implants incorporate integrated surgical guides that ensure precise screw positioning, optimising implant fixation and biomechanical stability. These built-in guides support intraoperative accuracy by directing screw trajectories along pre-planned paths, tailored to each patient’s bone quality and anatomy. This not only improves implant stability but also reduces surgical variability and the risk of complications.

Figure 4: The comparison between the standard (or conventional) and the new bone preserving strategies.

• Streamlined workflow and rapid turnaround
  The end-to-end process—from imaging and pre-operative planning to surgical implantation—is designed for efficiency, typically completed within six weeks. CT imaging is used to generate detailed anatomical models, which inform implant design and surgical planning. Following surgery, post-operative CT scans are used to confirm accurate implant positioning and successful restoration of joint biomechanics, providing immediate feedback on surgical outcomes.

  This integrated, patient-centred workflow represents a significant advancement in the field of orthopaedic reconstruction, particularly for patients with severe bone loss or complex anatomical challenges. By combining precision engineering with clinical insight, this approach enables safer, more predictable outcomes while preserving critical bone structure.

Conclusions

• Custom 3D-printed titanium implants are proving highly effective in reconstructing complex bone defects, offering enhanced stability, alignment, and integration with host bone.
• Osteolysis associated with metal-on-metal (MoM) implants can progress rapidly and become severe within a decade, highlighting the critical role of cross-sectional imaging—particularly CT scans—in assessing bone integrity when clinical concerns arise.
• Timely diagnosis and early intervention are essential. Identifying and managing osteolysis before it reaches an advanced stage enables more straightforward revision procedures and significantly improves surgical outcomes.

This patient-specific, image-led approach marks a significant advancement in the management of bone loss around hip replacements. It supports more conservative and tailored surgical strategies, ultimately contributing to safer revisions and better long-term outcomes for patients.

References

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3. Randelli F, Banci L, Favilla S, Maglione D, Aliprandi A. Radiographically undetectable periprosthetic osteolysis with ASR implants: the implication of blood metal ions. The Journal of arthroplasty. 2013;28(8):1259-64.
4. Heffernan E, Alkubaidan F, Nielsen Te, Munk P. The imaging appearances of metallosis. Skeletal radiology. 2008;37:59-62.
5. Waldstein W, Schmidt-Braekling T, Boettner F. MRI does not detect acetabular osteolysis around metal-on-metal Birmingham THA. Archives of orthopaedic and trauma surgery. 2014;134:1009-15.
6. Egawa H, Ho H, Huynh C, Hopper Jr RH, Engh Jr CA, Engh CA. A three-dimensional method for evaluating changes in acetabular osteolytic lesions in response to treatment. Clinical Orthopaedics and Related Research®. 2010;468(2):480-90.
7. Durand‐Hill M, Henckel J, Di Laura A, Hart AJ. Can custom 3D printed implants successfully reconstruct massive acetabular defects? A 3D‐CT assessment. Journal of Orthopaedic Research®. 2020;38(12):2640-8.
8. Di Laura A, Henckel J, Dal Gal E, Monem M, Moralidou M, Hart AJ. Reconstruction of acetabular defects greater than Paprosky type 3B: the importance of functional imaging. BMC Musculoskeletal Disorders. 2021;22(1):1-10.
9. Di Laura A, Henckel J, Wescott R, Hothi H, Hart AJ. The effect of metal artefact on the design of custom 3D printed acetabular implants. 3D Printing in Medicine. 2020;6(1):1-11.
10. Di Laura A, Henckel J, Hart A. Custom 3D-Printed Implants for Acetabular Reconstruction: Intermediate-Term Functional and Radiographic Results. JBJS Open Access. 2023;8(2).
11. Dzhavadov AA, Huang W, Li H, Noor SS, Parvizi J, Shahi A, et al. In which patients should a custom-made acetabular implant (triflange cup) be used? The Journal of arthroplasty. 2024:S0883-5403 (24) 01042-8.

Main image Figure 1: 3-Dimensional reconstruction of the bony pelvis from CT data showing a large acetabular defect (red arrows).

Image credits: Produced in house by the authors.

Staff Writer