Total hip arthroplasty (THA) in hips affected by developmental dysplasia is not a common and standard procedure, as it is often complex and difficult to perform. However, THA for a dysplastic hip in dwarfism can be an even more demanding and challenging procedure, in which small dimensions and bone dysmorphism are often the main problems. In some cases, even if accurate preoperative planning is performed, a standard prosthesis cannot be used, and custom-made devices are required. Currently, the biomedical industry has developed more advanced technologies that assist surgeons in designing and manufacturing innovative, custom-made implantable devices.
The authors present the case of a patient with dwarfism and bilateral developmental hip dysplasia who underwent THA with a bilateral custom-made trabecular titanium porous acetabular shell manufactured using selective laser melting to enhance osseointegration and bone ingrowth.
In November 2015, a 32-year-old man with dwarfism was referred to the authors' hospital because of disabling bilateral hip pain. The patient's height and body weight were 1.17 m and 30 kg, respectively. His dwarfism was harmonic and not hereditary.
The patient reported severe pain, mostly in the right hip, and significant limitations in activities of daily living. His right hip flexion/extension angle was 30°/0°; external/internal rotation angle, 20°/10°; and abduction/adduction angle, 10°/20°. His preoperative Harris Hip score was 10.
Standard radiographs revealed bilateral osteoarthritis secondary to developmental hip dysplasia of Crowe type II and II–III in the left and right hips, respectively (Figure 1A). A computed tomography (CT) scan was obtained, and a complete three-dimensional (3-D) digital reconstruction of the pelvis was created (Figure 1B). The CT scan revealed that the right and left acetabula had diameters of 34 mm and 37 mm, respectively, and lacked a posterior wall and superior roof. Bone thickness was less than 2 mm at the quadrilateral lamina.
Preoperative anteroposterior radiograph of the pelvis (A). Three-dimensional reconstruction of the pelvic computed tomography scan (B).
Owing to the acetabular morphology and the patient's young age, a cemented all-polyethylene cup was not chosen for implantation. Because the smallest-sized metal-backed shell commercially available was too large for a cementless implantation, a small-sized custom-made porous acetabular shell was mandatory to allow cementless THA with enhanced biologic fixation in this patient. For the femoral component, a customized stem was required because of the small size of the diaphyseal canal. Thus, the surgical strategy was to recreate the acetabular roof with bone grafting and to implant a cementless custom-made shell with an ultrahigh-molecular-weight polyethylene liner.
On the basis of the 3-D digital reconstruction of the pelvic CT image, a 30-mm-diameter shell with a unique cancellous bone-like highly porous trabecular titanium mesh was designed (Figure 2A). Selective laser melting, a laser additive manufacturing technology, was used to manufacture the implant in a continuous one-step process using a Ti6Al4V alloy powder. The trabecular titanium lattice (Figure 2B) had a 70% porosity with randomly irregular trabeculae (trabeculae mean roughness Ra, 2 µm) and fully interconnected pores with a mean size of 500 µm. The implant and its trabecular lattice, named Traser, were developed and manufactured by Permedica S.p.A. (Merate, Italy).
Custom-made trabecular acetabular shell created using selective laser melting (A). Scanning electron microscopy image of the titanium trabecular lattice (Traser; Permedica S.p.A., Merate, Italy) of the acetabular shell (B).
In April 2016, THA was performed on the right hip. Gentle acetabular reaming was performed using a custom-made 30-mm diameter reamer. A femoral head bone graft was fixed with 2 cancellous screws on the lateral roof. The shell then was implanted without cement and stabilized with 4 screws. A conventional electron beam-sterilized ultrahigh-molecular-weight polyethylene liner was used, coupled with a 17-mm diameter metal femoral head and a custom-made Wagner Cone-type stem as the femoral component.
Four months postoperatively, the patient was able to walk without limping and crutches. Range of motion was 120°/20° for flexion/extension, 60°/30° for external/internal rotation, and 40°/30° for abduction/adduction. Radiographically, the shell and bone graft were stable and showed complete osseointegration. In December 2016, the patient underwent THA on the left hip with the same procedure using a similar custom-made shell.
In May 2017, 1 year following the first THA, the patient's Harris Hip score was 100 on the right side, and function in both hips was completely restored. Radiographs showed an osseointegrated bilateral hip implant (Figure 3).
Anteroposterior radiograph of the pelvis obtained 1 year after total hip replacement of the right hip showing complete osseointegration of the selective laser melting custom-made bilateral trabecular acetabular shells.
Prosthetic joint replacement in patients with dwarfism can be particularly complex, even in primary procedures. In disproportionate and proportionate dwarfism with dysplastic hips, the normal bone shape is changed, and in some cases, custom-made prostheses are required as the result of the joint deformity and the small size of bones.1–3
In the current case of a young patient with proportionate dwarfism, the femur and pelvis were dysplastic and extremely reduced in size, with poor and thin acetabular sockets. Deep reaming would not resolve the absence of a complete acetabulum; therefore, autologous bone grafting reconstruction was necessary. Moreover, a customized prosthesis was required, as the size of the acetabulum required the use of a 30-mm acetabular shell. Customized prosthetic implants, such as acetabular cages and augments, are well known to be an effective alternative treatment to standard prostheses in cases of severe acetabular bone defects in complex THA.4,5
Currently, with computer-aided design and manufacturing as well as rapid prototyping techniques, complex custom-made implants are feasible, with a considerable amount of time saved from implant design to surgical implantation. Moreover, current additive manufacturing technologies enable the construction of prostheses with complex geometries that otherwise would be impossible to produce with traditional manufacturing tools. Selective laser melting is one of the more-advanced additive manufacturing technologies that can create a complex solid component with extremely high accuracy using a laser beam to melt patterns of a metal powder.6
Supported by these new technologies, a new generation of 3-D-printed custom-made implants is available as an option for complex surgical procedures such as revision THAs for severe bone defects7,8 or hip reconstructions after pelvic tumor resection.9 Selective laser melting has the ability to manufacture shells or cages of any shape and geometry that feature highly porous structures not coated onto the component but built up with the component itself in a unique piece. However, the custom-made acetabular implants reported in the literature showed a porous structure characterized by a repetitive regular network with pores of the same shape and size, which is different from the trabecular lattice of the bilateral custom-made shells described in the current case.
Selective laser melting could be a promising alternative technology for manufacturing highly porous, customized acetabular components for complex joint reconstruction procedures in which an enhanced ability to osseointegrate is particularly required.10
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- De Fine M, Traina F, Palmonari M, Tassinari E, Toni A. Total hip arthroplasty in dwarfism: a case report. Chir Organi Mov. 2008;92(1):67–69. doi:10.1007/s12306-008-0042-7 [CrossRef]
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- Sing SL, An J, Yeong WY, Wiria FE. Laser and electron-beam powder-bed additive manufacturing of metallic implants: a review on processes, materials and designs. J Orthop Res. 2016;34(3):369–385. doi:10.1002/jor.23075 [CrossRef]
- Colen S, Harake R, De Haan J, Mulier M. A modified custom-made triflanged acetabular reconstruction ring (MCTARR) for revision hip arthroplasty with severe acetabular defects. Acta Orthop Belg. 2013;79(1):71–75.
- Li H, Qu X, Mao Y, Dai K, Zhu Z. Custom acetabular cages offer stable fixation and improved hip scores for revision THA with severe bone defects. Clin Orthop Relat Res. 2016;474(3):731–740. doi:10.1007/s11999-015-4587-0 [CrossRef]
- Wong KC, Kumta SM, Geel NV, Demol J. One-step reconstruction with a 3D-printed, biomechanically evaluated custom implant after complex pelvic tumor resection. Comput Aided Surg. 2015;20(1):14–23. doi:10.3109/10929088.2015.1076039 [CrossRef]
- Wyatt MC. Custom 3D-printed acetabular implants in hip surgery: innovative breakthrough or expensive bespoke upgrade?Hip Int. 2015;25(4):375–379. doi:10.5301/hipint.5000294 [CrossRef]