Residual implanted material on radiographs or CT does not carry one universal meaning. In clinical follow-up, the question is usually straightforward: the graft or substitute is still visible, so what does that mean for healing and for the construct? A visible graft may be integrating, a scaffold may be behaving as designed, a fast-resorbing carrier may already have disappeared, or a concerning failure pattern may be developing.
The useful clinical question is not whether material remains visible, but what that visibility represents in that material class and whether the reconstruction is progressing towards union and mechanical stability.
The framework below is intended as a practical reading aid, illustrated with examples from the orthopaedic literature rather than a full review of every biomaterial class. The evidence base is stronger for some classes than for others, which is one reason a single visibility rule does not hold across all grafts and graft substitutes (Clatworthy et al., 2001; Kubosch et al., 2016; Park and Moon, 2018; Chuang et al., 2022).
Summary
Persistent material on imaging is not a diagnosis.
Different graft classes follow different radiographic pathways.
Structural allograft may remain visible while the host-graft junction unites.
Scaffold-type materials may remain visible because persistence is part of the material design.
Fast-resorbing materials can disappear before the defect has been replaced by bone.
Progressive lucency, resorption, migration, collapse, breakage, or nonunion carry more weight than visibility alone.
The central point
A visible graft on follow-up imaging is easy to overread in two opposite directions: as a sign that healing has failed, or as proof that the graft is still contributing in a useful way. Neither shortcut is reliable across orthopaedic biomaterials. Interpretation depends on the material class, the expected radiographic pathway, interval change, the interface, and mechanical behaviour. A surgeon looking at follow-up imaging is not asking one abstract question about persistence.
The useful question is: what material was implanted, how is it expected to behave on imaging, and is the reconstruction progressing in a mechanically and biologically convincing way? That is the framework used here. The examples below illustrate the principle; they do not claim equal evidence depth for every biomaterial class (Kubosch et al., 2016; Clatworthy et al., 2001; Park and Moon, 2018).
Why this matters clinically
Long-term reconstruction outcomes are reported through survivorship, union, stability, function, and revision, not through proof that every trace of implanted material has disappeared. In uncontained defects revised with structural allograft, ten-year allograft survival was 72% and overall success was 75%, with improved knee scores and range of motion (Clatworthy et al., 2001). In cortical strut allografting for periprosthetic bone defects, incorporation was reported in 18 of 19 cases, with a final Harris hip score of 93.3 and survivorship of 94.7% when nonunion and graft removal were used as endpoints (Park and Moon, 2018). In structural allograft reconstruction during two-stage exchange for knee periprosthetic joint infection, Knee Society Score improved from 33.1 to 75.4 and 8-year prosthesis survival was 90.9%, without a significant increase in relapse compared with matched controls (Chuang et al., 2022).
At the same time, follow-up imaging remains one of the main ways to assess how the implanted material and the surrounding reconstruction are behaving over time. That is why interpretation matters. A visible graft or graft substitute may fit with stable healing, delayed remodelling, scaffold persistence, or failure, depending on the material and the pattern over time. Imaging therefore has to be read in the context of construct performance rather than by visibility alone (Clatworthy et al., 2001; Park and Moon, 2018; Chuang et al., 2022).
An interpretive framework, illustrated with examples
1. Persistence can be compatible with healing
Persistent visible material does not automatically indicate failure. The strongest direct orthopaedic evidence for this comes from structural allograft and cortical strut allograft, where the graft may remain radiographically distinct while the host-graft junction incorporates and the construct remains functional.
This pattern has been described in:
structural allograft
cortical strut allograft
bioactive glass
composite graft substitutes
In structural allograft reconstruction, radiographic union has been defined by trabecular bridging and obliteration of the host-graft junction, and successful cases included persistent graft bulk with limited resorption rather than disappearance of the graft itself (Clatworthy et al., 2001). In cortical strut allografting, radiological incorporation was reported in 94.7% of cases over medium- to long-term follow-up, again without requiring disappearance of the strut as a success criterion (Park and Moon, 2018).
A similar interpretive issue appears in other material classes. Bioactive glass can remain visible as a dense region while healing proceeds, because the visible area may include residual material, newly mineralised tissue, or both (Heikkilä et al., 2011). Composite graft substitutes can also show mixed persistence because one phase disappears earlier while another remains longer (Iundusi et al., 2015; Nilsson et al., 2013).
Taken together, these examples support the same practical point: persistent density can still be compatible with healing when the construct is stable and the interval pattern is coherent for that material class.
2. Persistence may reflect scaffold design rather than failed healing
Another interpretive pattern appears in scaffold-type materials. In this setting, persistent visibility may reflect intended material behaviour rather than failed integration.
This pattern is relevant for:
hydroxyapatite-rich materials and ceramic-rich substitutes
calcium phosphate materials
Hydroxyapatite-rich materials and many ceramic-rich substitutes can remain radiopaque because slow remodelling is part of their material behaviour. In opening-wedge high tibial osteotomy, porous hydroxyapatite wedges remained radiographically dense across follow-up while histology showed direct bonding and regenerated bone in the pores (Koshino et al., 2001). More broadly, comparative orthopaedic work has emphasised that some graft substitutes do not disappear radiographically in the same way as allograft, which means radiographs and CT can struggle to distinguish retained substitute from absent bone ingrowth in every case (Kubosch et al., 2016).
Calcium phosphate materials illustrate the same issue in a different way, because this class is heterogeneous rather than radiographically uniform. Different formulations can show different resorption pathways, including relatively homogeneous reduction in opacity, peripheral resorption, or persistent radiopaque material with trabecular formation through the defect. Good functional results have been reported despite these different persistence patterns in tibial plateau fractures and metaphyseal osteotomies. The practical question is therefore not whether the substitute is still visible, but whether it is changing in the way expected for that formulation and continuing to support the reconstruction (Hanke et al., 2017; Winge and Røkkum, 2022; Hofmann, 2020).
3. Disappearance is not always reassuring
Alongside persistent visibility, disappearance of material does not prove that the defect has been replaced by structurally useful bone.
This pattern is relevant for:
cancellous graft remodelling
fast-resorbing carriers such as calcium sulphate
Experimental cancellous graft work showed that accelerated graft resorption can outpace new bone formation and leave more unmineralised tissue, which means loss of visible graft is not a universal success sign (Jensen et al., 2002). Surface-based remodelling can produce a mixed state in which interface incorporation is progressing while residual graft remains deeper in the construct. That is one reason radiographic continuity and complete biological replacement are not the same endpoint (Jensen et al., 2002).
The same caution applies to fast-resorbing carriers such as calcium sulphate. In that class, rapid disappearance may be expected, but disappearance alone does not establish bone fill. A lucent defect after calcium sulphate resorption can still represent incomplete healing, fibrous tissue, or insufficient structural restoration. The practical question is therefore not whether the carrier has vanished, but what has replaced it and whether the defect is regaining structural integrity (Mirzayan et al., 2001; Yu et al., 2009; Kumar et al., 2013).
4. The more important warning pattern is adverse change
Across material classes, the more concerning pattern is not simple persistence but adverse change over time. Progressive resorption associated with loosening, nonunion at the host-graft junction, migration, collapse, graft fracture, or component breakage carries more interpretive weight than visibility alone.
This pattern is seen in:
structural allograft revision knee reconstruction, where failure was linked to severe resorption causing implant loosening and to nonunion at the host-graft junction (Clatworthy et al., 2001)
two-stage exchange reconstruction for knee periprosthetic joint infection, where allograft resorption with tibial component breakage required revision (Chuang et al., 2022)
hydroxyapatite-rich interbody use, where fragmentation, collapse, and screw migration were the red flags rather than persistent material itself (McConnell et al., 2003)
A useful rule of thumb across the available evidence is to follow the pattern, the interface, and the mechanics. A stable visible graft or scaffold can be acceptable. A changing construct with lucency, collapse, migration, or loss of reduction deserves much more concern.
What this means for different material groups
For cancellous allograft, the relevant pattern is progressive loss of distinctness and trabecular continuity across the repair site rather than static persistence. Comparative orthopaedic literature has described incorporation through loss of graft radiodensity and trabecular crossing at the defect or fracture line (Kubosch et al., 2016).
For structural allograft and cortical strut allograft, persistent visible graft bulk can be entirely compatible with healing. The useful question is whether the host-graft junction is incorporating and whether the reconstruction remains stable (Clatworthy et al., 2001; Park and Moon, 2018; Chuang et al., 2022).
For DBM-containing grafts, persistence alone is less biologically specific on plain radiographs. A visible fusion mass may reflect the broader construct or later mineralisation rather than DBM itself. In this class, CT-defined bridging and convincing fusion progression carry more weight than persistence of a visible mass (NaPier et al., 2016; Kim et al., 2017; Bhamb et al., 2019).
For hydroxyapatite-rich materials, persistent density can be expected. The interpretation changes when persistence is accompanied by fragmentation, collapse, or loss of correction rather than bonding and maintained alignment (Koshino et al., 2001; McConnell et al., 2003).
For calcium phosphate materials, a formulation-specific approach is required because the class does not follow one radiographic pathway. Serial change and structural behaviour matter more than the presence of a dense residual mass on one image (Hanke et al., 2017; Winge and Røkkum, 2022; Hofmann, 2020).
For calcium sulphate materials, rapid disappearance can be expected, so the pitfall is reading resorption as healing without confirming bone replacement and restoration of structure (Mirzayan et al., 2001; Yu et al., 2009; Kumar et al., 2013).
For bioactive glass and composites, persistent density can reflect residual material, mineralised tissue, or both. Mixed-phase substitutes add further complexity because one component may disappear earlier while another remains longer (Heikkilä et al., 2011; Iundusi et al., 2015; Nilsson et al., 2013; van Dijk et al., 2020).
A practical reading sequence
This is an interpretive framework, not treatment advice.
A practical way to read follow-up imaging in this setting is to start with the material class, then compare the image with the expected pathway for that class, then assess interval change, and then judge the interface and the mechanical behaviour of the construct. That sequence is more useful than a binary visible-versus-not-visible reading (Kubosch et al., 2016; Clatworthy et al., 2001; Park and Moon, 2018; Chuang et al., 2022).
What not to assume from visibility alone, according to the evidence
A visible graft is an unhealed graft. This is false across several classes, especially structural allograft and scaffold-type materials (Clatworthy et al., 2001; Park and Moon, 2018; Koshino et al., 2001).
A disappeared graft has been replaced by bone. This is not a safe assumption, especially in fast-resorbing carriers and in situations where resorption can outpace useful bone formation (Jensen et al., 2002; Mirzayan et al., 2001).
All ceramics behave the same way. Calcium phosphate materials, hydroxyapatite-rich materials, bioactive glass, and composites do not share one radiographic fate (Hanke et al., 2017; Heikkilä et al., 2011; Nilsson et al., 2013).
One radiograph is enough to decide. Serial imaging, interface assessment, and construct behaviour are more informative than a single time point (Kubosch et al., 2016; Winge and Røkkum, 2022).
Closing note
Persistent graft or graft substitute material on follow-up imaging is a material-specific radiographic finding, not a diagnosis. The practical framework is straightforward: identify the material class, understand the expected radiographic pathway, assess interval change, judge the interface, and pay close attention to mechanical behaviour. That approach is more clinically useful than asking whether the material is still visible. The examples above illustrate the framework; they do not claim that the full orthopaedic biomaterials literature is uniform or complete.
The core point remains: there is no single rule for persistent graft visibility across orthopaedic biomaterials (Clatworthy et al., 2001; Jensen et al., 2002; Kubosch et al., 2016; Park and Moon, 2018; Chuang et al., 2022).
References
Bhamb N, Kanim LEA, Drapeau S, Mohan S, Vasquez E, Shimko D, McKay W, Bae H. Comparative Efficacy of Commonly Available Human Bone Graft Substitutes as Tested for Posterolateral Fusion in an Athymic Rat Model. Int J Spine Surg. 2019;13(6):544-558. PMID: 31745449.
Chuang CA, Lee SH, Chang CH, Hu CC, Shih HN, Ueng SWN, Chang Y. Application of structural allogenous bone graft in two-stage exchange arthroplasty for knee periprosthetic joint infection: a case control study. BMC Musculoskelet Disord. 2022;23(1):325. doi:10.1186/s12891-022-05228-6. PMID: 35382827.
Clatworthy MG, Ballance J, Brick GW, Chandler HP, Gross AE. The use of structural allograft for uncontained defects in revision total knee arthroplasty. A minimum five-year review. J Bone Joint Surg Am. 2001;83(3):404-411. PMID: 11263645.
Heikkilä JT, Kukkonen J, Aho AJ, Moisander S, Kyyrönen T, Mattila K. Bioactive glass granules: a suitable bone substitute material in the operative treatment of depressed lateral tibial plateau fractures: a prospective, randomized 1 year follow-up study. J Mater Sci Mater Med. 2011;22(4):1073-1080. doi:10.1007/s10856-011-4272-0. PMID: 21431354.
Hanke A, Bäumlein M, Lang S, Gueorguiev B, Nerlich M, Perren T, Rillmann P, Ryf C, Miclau T, Loibl M. Long-term radiographic appearance of calcium-phosphate synthetic bone grafts after surgical treatment of tibial plateau fractures. Injury. 2017;48(12):2807-2813. doi:10.1016/j.injury.2017.10.030. PMID: 29096930.
Hofmann A. Autologous Iliac Bone Graft Compared with Biphasic Hydroxyapatite and Calcium Sulfate Cement for the Treatment of Bone Defects in Tibial Plateau Fractures: A Prospective, Randomized, Open-Label, Multicenter Study. J Bone Joint Surg Am. 2020;102(3):179-193. PMID: 31809394.
Iundusi R, Gasbarra E, D'Arienzo M, Piccioli A, Tarantino U. Augmentation of tibial plateau fractures with an injectable bone substitute: CERAMENT™. Three year follow-up from a prospective study. BMC Musculoskelet Disord. 2015;16:115. doi:10.1186/s12891-015-0574-6. PMID: 25968241.
Jensen TB, Overgaard S, Lind M, Rahbek O, Bünger C, Søballe K. Osteogenic protein 1 device increases bone formation and bone graft resorption around cementless implants. Acta Orthop Scand. 2002;73(1):31-39. PMID: 11928908.
Kim BJ, Kim SH, Lee H, Lee SH, Kim WH, Jin SW. Demineralized Bone Matrix (DBM) as a Bone Void Filler in Lumbar Interbody Fusion: A Prospective Pilot Study of Simultaneous DBM and Autologous Bone Grafts. J Korean Neurosurg Soc. 2017;60(2):225-231. doi:10.3340/jkns.2017.0101.006. PMID: 28264244.
Koshino T, Murase T, Takagi T, Saito T. New bone formation around porous hydroxyapatite wedge implanted in opening wedge high tibial osteotomy in patients with osteoarthritis. Biomaterials. 2001;22(12):1579-1582. PMID: 11374457.
Kubosch EJ, Bernstein A, Wolf L, Fretwurst T, Nelson K, Schmal H. Clinical trial and in-vitro study comparing the efficacy of treating bony lesions with allografts versus synthetic or highly-processed xenogeneic bone grafts. BMC Musculoskelet Disord. 2016;17:77. doi:10.1186/s12891-016-0930-1. PMID: 26873750.
Kumar YC, Nalini KB, Menon J, Patro DK, Banerji BH. Calcium sulfate as bone graft substitute in the treatment of osseous bone defects, a prospective study. J Orthop Surg (Hong Kong). 2013;21(1):124-127.
McConnell JR, Freeman BJC, Debnath UK, Grevitt MP, Prince HG, Webb JK. A prospective randomized comparison of coralline hydroxyapatite with autograft in cervical interbody fusion. Spine (Phila Pa 1976). 2003;28(4):317-323. doi:10.1097/01.BRS.0000048503.51956.E1. PMID: 12590203.
Mirzayan R, Panossian V, Avedian R, Forrester DM, Menendez LR. The use of calcium sulfate in the treatment of benign bone lesions. A preliminary report. J Bone Joint Surg Am. 2001;83(3):355-358. doi:10.2106/00004623-200103000-00006. PMID: 11263638.
NaPier Z, Kanim LEA, Thordarson S, Kropf MA, Cuéllar JM, Glaeser JD, Bae HW. Demineralized Bone Matrix Bone Biology and Clinical Use. Semin Spine Surg. 2016;28(4):196-216. doi:10.1053/j.semss.2016.08.003.
Nilsson M, Zheng MH, Tägil M. The composite of hydroxyapatite and calcium sulphate: a review of preclinical evaluation and clinical applications. Expert Rev Med Devices. 2013;10(5):675-684. doi:10.1586/17434440.2013.827529. PMID: 24053255.
Park JS, Moon KH. Medium- to Long-term Results of Strut Allografts Treating Periprosthetic Bone Defects. Hip Pelvis. 2018;30(1):23-28. doi:10.5371/hp.2018.30.1.23. PMID: 29564294.
van Dijk LA, Barrère-de Groot F, Rosenberg AJWP, Pelletier M, Christou C, de Bruijn JD, Walsh WR. MagnetOs, Vitoss, and Novabone in a Multi-endpoint Study of Posterolateral Fusion: A True Fusion or Not? Clin Spine Surg. 2020;33(6):E276-E287. doi:10.1097/BSD.0000000000000920. PMID: 31977334.
Winge MI, Røkkum M. Calcium phosphate bone cement and metaphyseal corrective osteotomies in the upper extremity: long-term follow-up of 10 children. Acta Orthop. 2022;93:769-774. doi:10.2340/17453674.2022.4589. PMID: 36161337.
Yu B, Han K, Ma H, Zhang C, Su J, Zhao J, Li J, Bai Y, Tang H. Treatment of tibial plateau fractures with high strength injectable calcium sulphate. J Biomed Mater Res B Appl Biomater. 2009;90(1):117-127. PMID: 18704416.