Chapter 12: Bone Substitutes and Implants · Bone Healing & Grafting CME

12.1 Introduction

This chapter describes alloplastic materials: synthetic materials that are not bone.

They are used mainly to replace shape in areas that do not carry much load: the skull (cranioplasty), the orbital walls, and facial contour (malar, chin, mandibular angle).

The main point of this chapter: an alloplastic implant has no blood supply and never gets one. It cannot fight infection. Its survival depends on the soft tissue covering it.

Flowchart: Alloplastic materials

Alloplastic materials
- Metal
  - Titaniummesh or custom plate; osseointegrates
- Polymers
  - PEEKcustom implant
  - PMMAbone cement
  - Porous polyethylenefacial implants
- Ceramics
  - Hydroxyapatite
  - Calcium phosphate cement
  - Bioactive glass

Used for: cranioplasty (12.3), orbital reconstruction (12.4), facial contour (12.5).

12.2 The materials

Material	Relationship with bone	How it is shaped	Advantages	Disadvantages
Titanium (mesh or custom plate)	Osseointegrates	Bent in theatre, or custom-made from CT	Strong, thin, tolerates contamination better than others	Can be visible or exposed under thin scalp; artifact on CT
PEEK (custom implant)	No bonding; fibrous capsule	Custom-made from CT	Stiffness similar to bone; exact fit; no CT artifact	No integration; infection; cost
PMMA (bone cement)	No bonding; fibrous capsule	Molded in theatre	Cheap, widely available	Heat during setting; brittle; infection
Hydroxyapatite and calcium phosphate cement	Bonds to bone; osteoconductive for a few millimeters	Paste that sets	Good for small contour defects	Brittle; fragments in large defects; infection near sinuses
Porous polyethylene (Medpor)	Soft tissue grows into the pores	Carved with a blade; fixed with screws	Facial contour, orbital floor	Exposure under thin cover; difficult to remove
Bioactive glass, porous ceramics	Bonds to bone	Granules, putty or blocks	Filling contained defects	Brittle; no strength

(Gosain 2005; Rubin and Yaremchuk 1997)

12.2.1 Further details

Property	Detail
Elastic modulus	Cortical bone 15 to 20 GPa; PEEK 3 to 4 GPa; PMMA 2 to 3 GPa; titanium about 110 GPa
Calcium phosphates	Hydroxyapatite (HA): very slow resorption, lasts for years. β-tricalcium phosphate (β-TCP): resorbs in months. Biphasic calcium phosphate mixes the two. Calcium sulfate: resorbs in weeks.
PMMA	Sets by an exothermic reaction that can exceed 70 °C; irrigate while it sets and protect the dura. Can be mixed with antibiotics.
Porous polyethylene	Pores of roughly 100 to 250 µm allow fibrovascular ingrowth, which fixes the implant and may reduce infection risk.
Bioactive glass	Silica, calcium, sodium and phosphate. In tissue fluid the surface forms a hydroxyapatite layer that bonds to bone. Has some antibacterial effect.
Custom implants	CT → 3D model of the defect (mirrored from the normal side) → implant designed → milled or 3D printed in titanium or PEEK

**Titanium mesh.** © AO Foundation, AO Surgery Reference.

**From CT scan to CAD plan to printed titanium implant.** A cranial defect is traced from a CT scan (left), a matching implant is designed in CAD software (centre), and the plan is printed in titanium (right). Photo: Maikel Beerens, Xilloc, Wikimedia Commons, CC BY-SA 4.0

**Hydroxyapatite graft material in tissue.** Hydroxyapatite may come from coral, bovine bone or algae, or be made synthetically. It is an acellular scaffold beside host tissue. Photo: Atlas of Medical Foreign Bodies, Wikimedia Commons, CC BY-SA 2.0

**Bioactive glass particles.** A rough, high-surface-area particle. Composition sets how its surface reacts and dissolves. Photo: Oregon State University, Wikimedia Commons, CC BY-SA 2.0

**Bone substitute granules being placed in a defect.**

Bioactive glass granules delivered from an applicator — **Bioactive glass granules (S53P4).** The granules are delivered from an applicator into a bone cavity (shown on a model). Inset: granule and putty forms. It is used, for example, to fill the cavity left after debridement of chronic osteomyelitis. Claireaux et al., Bone Jt Open 2025, Fig. 1, CC BY 4.0; image courtesy of Bonalive Biomaterials.

Calcium sulfate bead kit, mould and finished beads — **Calcium sulfate beads.** The kit contains calcium sulfate powder, mixing solution and a flexible mould (top). The paste is mixed with antibiotic powder and spread into the mould. The set beads are pushed out (bottom) and packed into the cavity. They dissolve in weeks and release the antibiotic locally. Cursaru et al., Life 2024, Fig. 1, CC BY 4.0.

Beta-tricalcium phosphate block in an osteotomy gap — **β-tricalcium phosphate block.** A block of β-TCP (arrow) and smaller blocks of another calcium phosphate (arrowhead) fill the gap of a pelvic osteotomy. Ceramic blocks resist compression but are brittle and must be protected by fixation. Fukuchi et al., Cureus 2025, Fig. 2, CC BY 4.0.

Porous polyethylene implants adapted on a printed skull model — **Porous polyethylene shaped on a printed model.** The implants become malleable in hot saline. Here they are shaped against a printed model of the patient before surgery. Heck et al., Braz Dent J 2026, Fig. 4, CC BY 4.0.

**Porous polyethylene implants** (manufacturer's catalogue image).

12.2.2 Examples of products

Material	Examples	Company
Titanium	Mesh and plates: MatrixNEURO, MatrixORBITAL, MatrixMANDIBLE. Similar mesh, preformed orbital plates and custom implants are made by other companies.	DePuy Synthes; KLS Martin; Stryker; Medartis
PEEK	Custom implants milled from PEEK-Optima polymer, made to order from the CT scan	DePuy Synthes; KLS Martin; Stryker
PMMA	Bone cement mixed in the operating room: Palacos, Simplex P. HTR-PMI is a custom porous implant based on PMMA.	Heraeus; Stryker; Zimmer Biomet
Porous polyethylene	Medpor: sheets and preformed shapes for the orbit, malar area, chin, mandibular angle and ear. Medpor Titan has titanium mesh inside. Others: Su-Por, Synpor.	Stryker (first made by Porex); Poriferous; DePuy Synthes
Calcium phosphate cement	Norian; BoneSource; HydroSet; Mimix	DePuy Synthes; Stryker; Stryker; Zimmer Biomet
Hydroxyapatite	Pro Osteon (made from coral); GranuMaS (granules made in Malaysia)	Zimmer Biomet; GranuLab
β-tricalcium phosphate	Vitoss; chronOS; Cerasorb; Osferion	Stryker; DePuy Synthes; curasan; Olympus Terumo Biomaterials
Biphasic calcium phosphate	MBCP; Mastergraft	Biomatlante; Medtronic
Calcium sulfate	Osteoset; Stimulan (often mixed with antibiotics)	Wright Medical; Biocomposites
Bioactive glass	NovaBone; PerioGlas; BonAlive (S53P4)	NovaBone Products; Bonalive Biomaterials
Resorbable plates and mesh	LactoSorb; RapidSorb; SonicWeld Rx; Inion CPS	Zimmer Biomet; DePuy Synthes; KLS Martin; Inion
Polycaprolactone mesh	Osteomesh: a 3D-printed resorbable mesh for the orbital floor and burr holes	Osteopore (Singapore)
Silicone	Solid chin, malar and nasal implants. "Silastic" was the original trade name.	Implantech and others
ePTFE	Gore-Tex sheets and facial implants; no longer marketed for the face	W. L. Gore

12.3 Cranioplasty

The commonest situation is a large skull defect after decompressive craniectomy. There are two choices: put back the patient's own stored bone flap, or use an alloplastic implant.

Indications for cranioplasty: protection of the brain; cosmetic contour; and treatment of the "syndrome of the trephined" (sinking skin flap syndrome): headache, dizziness, mood change and neurological deficit that improve after cranioplasty.

Timing: traditionally at least 3 months after craniectomy, and 6 to 12 months after an infection. Earlier cranioplasty is increasingly practiced in clean cases. The scalp must be healthy and relaxed enough to close without tension.

12.3.1 The stored autologous bone flap

The patient's own bone flap seems the best choice. But after storage it is a large, dead, non-vascularized cortical graft. It often resorbs.

Study	Finding
Grant 2004: 40 children and adolescents	50% had bone flap resorption that needed reoperation
Honeybul 2017: randomized trial, 64 adults	22% of autologous bone flaps resorbed completely within 1 year. No failures with custom titanium. Total cost was not different.
Malcolm 2018: meta-analysis, 1,586 cranioplasties	Autologous bone had 1.9 times more reoperations than synthetic materials. Resorption in 20%. Infection rate was not different.

So resorption is the main problem with autologous bone flaps, especially in children and young adults.

Flowchart: Cranioplasty after craniectomy

Skull defect
- Infant under about 1 year
  - Often closes by itself, or use bone graft
- Child
  - Split calvarial graftstored flap resorbs in about 50%
- Adult
  - Good bed
    - Stored bone flapresorbs in about 20%
    - Custom titanium or PEEK
  - Poor bed: infected, irradiated, thin scalp, near sinus
    - Improve soft tissue cover first, then autogenous bone or titanium mesh

12.3.2 Does the choice of alloplastic material matter?

Lindner (2017) randomized 52 patients to custom hydroxyapatite or custom titanium. Infection was 7.7% versus 20.8% (not statistically significant). About 28% in each group needed reoperation.

The reoperation rate was high with both materials. In a long-term series of 312 patients, complications occurred in 23.6% (Moreira-Gonzalez 2003). This suggests that the condition of the bed is at least as important as the material.

Patient-specific PEEK cranial implant — **Custom PEEK cranial implant.** Made by computer-aided design and milling from the CT scan. The holes allow drainage and suture of the dura and temporalis. Falguera Uceda et al., Pharmaceutics 2022, Fig. 2, CC BY 4.0.

**PEEK cranioplasty with free flap cover.** A: cutting guide for the skull resection. B: the PEEK implant fixed with miniscrews. C: latissimus dorsi flap. D and E: result. The implant was placed only with vascularized soft tissue cover. Rana et al., Craniomaxillofac Trauma Reconstr 2025, Fig. 3, CC BY 4.0.

PMMA cement prepared in a two-piece mould — **PMMA cranioplasty made in a mould.** A two-piece mould is printed from the CT scan. PMMA cement is mixed in theatre and pressed in the mould. This gives a custom implant at low cost. da Silva Júnior et al., 3D Print Med 2021, Fig. 8, CC BY 4.0.

PMMA cranioplasty being placed on the skull defect — **Placing the PMMA implant.** The moulded implant is brought onto the defect and fixed with sutures or plates. Scerrati et al., Int J Med Robot 2021, Fig. 2, CC BY 4.0.

12.3.3 Choosing the material

The choice depends on the patient's age, the condition of the bed and the size of the defect:

Situation	Usual choice	Reason
Infant under about 1 year	Observe, or autogenous bone	The dura can still form bone; fixed implants restrict growth and can migrate
Child	Split calvarial graft; custom implant if bone is insufficient	Stored bone flaps resorb in about half of children
Adult, clean bed, small defect	Split calvarium, titanium mesh or cement	All work in a good bed
Adult, clean bed, large defect	Custom titanium or PEEK; or the stored bone flap	Stored flap resorbs completely in about 1 in 5
Next to the frontal sinus	Autogenous bone; separate the sinus first (pericranial flap)	Implants and cements become infected
Previously infected	Delay; then autogenous bone or titanium mesh under good cover	Avoid cements and the old infected flap
Irradiated or thin scalp	Improve the scalp first (flap or tissue expansion), then reconstruct	An implant under poor cover will be exposed

Based on Paige 2006; Grant 2004; Honeybul 2017; Malcolm 2018; Moreira-Gonzalez 2003; Rubin and Yaremchuk 1997.

Titanium mesh cranioplasty after removal of a skull tumor — **Titanium mesh cranioplasty.** Left: a skull lesion exposed through a craniotomy. Right: after resection, the bone defect is covered with titanium mesh fixed with screws (lower arrow). Zhang et al., Front Oncol 2026, CC BY 4.0.

12.4 Orbital reconstruction

The orbital floor and medial wall do not carry load. The implant only needs to restore the shape of the orbit and hold it.

Ellis and Tan (2003) compared titanium mesh and cranial bone graft in 58 blow-out fractures, measured on CT. Titanium mesh restored the orbital shape more accurately.
Options: titanium mesh, porous polyethylene sheet, resorbable sheet, or bone graft.
Bone graft is still preferred when the soft tissue cover is thin, when there is open communication with a sinus, or when the bed has been infected or irradiated.

**Preformed titanium orbital plates.** © AO Foundation, AO Surgery Reference.

**Orbital floor implant on postoperative CT.** © AO Foundation, AO Surgery Reference.

Titanium mesh being placed on the orbital floor — **Titanium mesh for an orbital floor fracture.** (a) The mesh is shaped and fixed to the orbital rim through a lower eyelid incision. (b) Radiograph after surgery showing the mesh on the right orbital floor (arrow). Ramanujam et al., Cureus 2023, CC BY.

12.5 Facial contour implants

Porous polyethylene implants are widely used for the malar area, chin and mandibular angle. Yaremchuk (2003) described the principles:

Place the implant directly on bone, under the periosteum.
Fix it with screws, so that it does not move and there is no gap under it.
Smooth the edges so that it cannot be felt.
Close healthy soft tissue over it, with the incision away from the implant.

These are the same principles as for an onlay bone graft: contact, fixation and good cover.

12.6 Why implants fail

Implant failure is nearly always a problem of the bed and the soft tissue cover.

No blood supply. Antibiotics and white cells reach bone through its vessels. They cannot reach the surface of an implant. Bacteria on the implant form a biofilm that is very hard to clear.
Thin or tight soft tissue cover. Pressure from the implant thins the skin or mucosa until the implant is exposed.
Communication with a contaminated space (frontal sinus, nose, mouth).
Previous infection or radiotherapy.

An exposed bone graft can sometimes be saved, because the bed can revascularize it. An exposed implant usually has to be removed.

Implants can also fail slowly. A smooth implant that is not fixed can move on the bone. Pressure and movement cause the bone under it to resorb.

**Bone resorption under a silicone chin implant.** A and B: radiograph and CT four years after placement, showing a deep defect in the chin under the implant, close to the incisor roots. C: the defect after removal. D: the removed implant. Brito et al., Craniomaxillofac Trauma Reconstr 2025, Fig. 3, CC BY 4.0.

Flowchart: Order of work before placing an implant

Control infection
Vascularized soft tissue cover
Tension-free closure
Choose the material

Order of work. First control infection. Second, provide good vascularized soft tissue cover (local flap, tissue expansion or free flap). Third, close without tension. Only then choose the implant material. In a poor bed, use autogenous or vascularized tissue instead of an implant.

Exposed titanium mesh on the forehead, and its treatment with a new mesh and a free flap — **An exposed implant and its treatment.** (A) Titanium mesh exposed through thin forehead skin after cranioplasty. (B) A new custom mesh. (C) The mesh is replaced and reshaped. (D) The scalp defect is covered with a free anterolateral thigh flap. (E, F) Later result. The problem was the soft tissue cover, and the solution was a flap. Wang et al., Medicine 2023, CC BY 4.0.

12.7 Summary

Alloplastic materials replace shape, not living bone.
Only titanium and the calcium-based materials bond to bone.
The stored bone flap after craniectomy often resorbs; a custom implant is a valid alternative in adults.
The soft tissue cover decides whether an implant survives.

References

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Honeybul S, Morrison DA, Ho KM, Lind CRP, Geelhoed E. A randomized controlled trial comparing autologous cranioplasty with custom-made titanium cranioplasty. J Neurosurg 2017;126:81–90.
Malcolm JG, Mahmooth Z, Rindler RS, et al. Autologous cranioplasty is associated with increased reoperation rate: a systematic review and meta-analysis. World Neurosurg 2018;116:60–68.
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