Total Hip Arthroplasty in Patients With Dwarfism

Abstract

Skeletal dysplasia or dwarfism presents in a host of manners. Degenerative hip disease can present as a primary problem secondary to the abnormal growth disturbance or secondarily from the abnormal load distributions through the hip joint itself. Total hip arthroplasty is a successful procedure but sought with increased risks and complications when compared to routine hip arthroplasty. Custom or modular hip implants can help a surgeon manage the abnormal bone morphology seen with this condition.

An orthopaedic surgeon may treat a host of skeletal dysplasias. By definition, dwarfism is a disproportionate shortness of stature, as compared to midgets, whose body proportions are normal.¹ These terms have developed discriminatory connotations, leading to the generic lay term of little people. Each dwarfism condition has a varying effect on a patient’s bones and joints resulting in varied deformities and secondary degenerative changes. Limb deformities are prevalent in short-limb dwarfs such as achondroplastics or pseudoachondroplastics, whereas other conditions can severely affect the axial skeleton, producing neurological sequelae.

Because of the abnormal morphology of the affected joint, biomechanical forces transmitted through the joint are abnormal in both direction and quantity, and cause excessive compression forces, such as weight-bearing, while creating further tension loads away from the joint line. With asymmetric stressing of the growth plate, deformity, secondary ligamentous laxity, and secondary degenerative changes result.¹ These degenerative changes, which are present later in life, can be avoided if the deformities and malalignment are corrected. Disruption of ossification centers can occur by alteration in a patient’s blood supply and by abnormal cartilage development.^1,2

Kopits¹ described the orthopedic complications of dwarfism. Awareness of associated deformities is critical to ensuring that patients are symptomatic from the affected joint and not from radiating symptoms. Associated deformities must also be noted to ensure that a patient is able to manage the rehabilitative phase. Upper extremity abnormalities and short statures mandate that walking assists be accommodating. Additionally, reaching assists, beds, and toileting facilities require preoperative planning to ensure appropriateness and availability.

During the intraoperative phase, cervical spine anomalies, including odontoid hypoplasia, must be managed prudently to avoid cord damage. Thorough preoperative radiographic documentation of cervical anomalies is essential with awake endoscopic intubation being critical during operative procedures.

Achondroplasia is inherited by autosomal dominant transmission. Infants exhibit a thoracolumbar kyphosis with extreme laxity of the knee joints, whereas toddlers demonstrate a variable bowleg deformity. The most significant problem of neurologic compromise results from spinal stensosis. Congenital hip dysplasia is never encountered with secondary osteoarthritis of the knee or hip, and is rarely described before age 40.

Pseudoachroplastic dysplasia has a host of autosomal dominant and recessive inheritances. Scoliosis is of moderate severity. Facial features are normal, but the condition has severe ligamentous laxities and limb deformities. When the patient is a toddler, deformities are demonstrated with windswept extremities causing the adducted hip to sublux. These windswept deformities become even more severe in the teenage years and result in a dysplastic hip.

Diastrophic dwarfism is autosomal recessive. Patients experience severe musculoskeletal deformities including hitchhiker thumbs, clubfeet, and flexion deformities of the hips, knees, and elbows. Spinal deformities are severe and progress. Patients’ hips are misshapen and undergo secondary degenerative change at or before skeletal maturity. As hips sublux laterally and the femoral head “pancakes” out, posterior acetabular deformities can be significant.

Patients with metaphyseal chondrodysplasia experience gradual, but progressive, bowing of the long bones. Their hips can show signs of coxa vara and respond to corrective osteotomies.

Morquio syndrome, or mucopolysaccharidosis, is associated with system conditions including corneal opacities and aortic valve damage. Shortness of stature becomes evident at age 2, but with regular proportions. Joint laxity is severe and the spine growth is more severely affected than the limbs. By age 7 or 8, upper motor neuron deficits develop from cervical spine myelopathy. Patients can lose the ability to walk by their teens. Hip dislocation is common, but only problematic if myelopathic issues do not take precedence.

Figure 1: A 55-year-old man with achondroplasia and secondary degenerative hip changes. Offset and length markedly increased with hip arthroplasty (A). Modular hip stems accommodated abnormal femoral version and bowing at 4-years’ follow-up (B).

Spondyloepiphyseal dysplasia congenita is usually transmitted by autosomal dominant inheritance. Infants display normal intelligence with some hypotonia early. By age 5, body proportion abnormalities become evident. Spine development lags behind the limbs giving a similar appearance to patients with Morquio syndrome. Coxa vara is observed and leads to dislocation if not managed. In addition, bowleg deformities are moderate.

Goals of total hip arthroplasty are similar with dwarfs as with patients with primary arthritis. It is difficult, however, to achieve these goals as implants are not accommodating to the associated deformities. Functional improvement is, however, significant in patients with primary arthritis.^3-6 This improvement is sought with increased associated risks and complications.^7,8

Arthroplasty technique and implants also require modification. Traditional implants have significant drawbacks. Offset and neck lengths are too excessive for small patients (Figure 1).³ Implants with too much offset or length can induce undue stretch on the sciatic nerve leading to palsies. An imbalanced tight hip can also cause secondary spinal deformities or pelvic obliquities producing secondary pain centers in other joints or the lumbosacral spine region. An imbalanced hip, which already has ligamentous laxity, is also prone to dislocation.⁷

A traditional femoral implant spans the femoral isthmus in a dwarf. An implant that is too long will not accommodate the excessive femoral bowing (Figure 2) and can create a conflict between the implant and the femoral diaphysis leading to fracture. Further, fixation, which is achieved at or past the isthmus of the femur, is excessive and can make revision procedures complicated and catastrophic as significant bone loss, in an already short femur, can occur. Version is also excessive in the femur. Implants that cannot accommodate the deformed metaphysis lead to calcar cracks. This prompted Huo et al³ to design a custom cemented femoral prosthesis. Most patients in their study had congenital dislocations, but six patients had skeletal dysplasias. A 93% probability of survival existed at 9 years with 95% excellent results. No fractures, nerver palsies, or perforations were reported.


Figure 2: A 42-year-old woman with pseudoachondroplasia and severe anterior femoral bowing (A). A transected femoral implant was inserted to accommodate femoral bowing (B).

The dwarf acetabulum is commonly shallow with significant version abnormalities. Commonly, a dysplastic acetabulum with deficient anterior walls is present, but one also needs to be aware of the possible deficient posterior walls as seen with diastrophic dwarfs. Augmenting acetabular fixation with screws is critical. Optimizing polyethylene thickness is also essential to limiting wear. Alternative bearing surfaces may be an alternative in these small diameter cups, as wear with traditional bearings is severe.⁷

Researchers at the Mayo Clinic reported on a series of 60 arthroplasties performed in patients with dwarfism.⁷ At the time of the average 12.8-year follow-up, 30% of patients required revision, with the majority of patients revised for aseptic loosening and osteolysis. Periprosthetic fracture occurred in two of the patients at the time of insertion of the implant, with an additional three patients experiencing fracture later in the follow-up phase. Twenty-five percent of patients’ femoral implants were malpositioned. Dislocations occurred in three of the patients’ hips despite adequate positioning of the component. The researchers’ conclusion was to consider use of a custom implant to avoid such difficulties. Peltonen et al⁶ found similar loosening rates in their series of 14 patients.

Custom implants are not optimal for most surgeons. The author advocates the use of a modular hip system that manages all the previously noted difficulties. The S-ROM system (DePuy Orthopaedics Inc., Warsaw, Ind) has been incorporated into practice for patients with challenging anatomy. Small acetabular shells used in developmental dysplastic hips are also used for patients as fixation can occur with a porous coating and augmented with screws. Small shells can now accommodate metal-on-metal articulations to potentially reduce particulate wear.

A modular femoral stem has significant advantages over traditional monoblock femoral stems. Small sized stems have short neck lengths, adjustable offsets, and small diameters, as they are commonly used for hip dysplasia.⁸ When using these smaller sized stems, surgeons avoid an unbalanced hip and potentially diminish complications. With stems being modular from the body, version can be managed independently from fixation. Severe torsion of the femur exists in many of these patients, so independent adjustment of implant fixation and version is critical. Fixation of the body with this system occurs proximally, avoiding distal fixation and the previously discussed concerns. To accommodate the significant bowing in the dwarf femur, stems are transected. Patients are made aware of the off-label use of the stem. Because the stems are titanium, transaction is accommodated with a metal cutting burr. Stem shortening avoids fracture risk or the need of morbid corrective osteotomies.

The author has retrospectively reviewed 14 dwarfs who experienced stem shortening with a minimum 2-year follow-up, average 4.3 years. All implants obtained ingrowth with no evidence of radiographic migration. No periprosthetic fractures have occurred to date. One dislocation occurred and was corrected with adjusting version of the femoral stem independently of the ingrowth femoral body sleeve. Anteversion of the femoral stem in excess of 60° was required to obtain stability of the dislocated joint. No further dislocations or other revisions have occurred.

Few outcome studies of hip arthroplasty for degenerative changes in the dwarf population exist.^3,6,7 Studies have shown significant functional improvement with the procedure but with risks and complications greater than with other primary hip arthroplasty groups.^7,8 The author advocates the use of a modular hip system to ensure an appropriate balanced hip arthroplasty. Position of components must accommodate the anatomy, and this means positioning components out of the range of traditional “acceptable” positions (Figure 2). Even with these components, the length of the component is excessive in short-stature patients. All of the author’s stems required stem tip transaction to avoid component-bone conflict (Figure 1). Finally, the role of alternative bearings must be assessed. Previous studies have shown significant wear and lysis related to increased activity levels, increased joint reactive forces, and relatively small components.⁷ A harder bearing surface may avoid this risk.

Hip arthroplasty is effective and predictable in patients with dwarfism. However, significant modifications from traditional primary arthroplasty techniques must be considered to ensure success.

References

Kopits S. Orthopaedic complications of dwarfism. Clin Orthop. 1976; 114:179.
Vaara P, Peltonen J, Poussa M, et al. Development of the hip in diastrophic dysplasia. J Bone Joint Surg Br. 1998; 80:315-320.
Huo MH, Salvati EA, Liberman JR, Burstein AH, Wilson PD Jr. Custom-designed femoral prostheses in total hip arthroplasty done with cement for severe dysplasia of the hip. J Bone Joint Surg Am. 1993; 75:1497-1504.
Bell RS, Rosenthal RE. Bilateral total hip replacement in a diastrophic dwarf. Orthopedics. 1980; 3:534-536.
Wirtz DC, Birnbaum K, Siebert CH, Heller KD. Bilateral total hip replacement in pseudoachondroplasia. Acta Orthop Belg. 2000; 66:405-408.
Peltonen JL, Hoikka V, Poussa M, Paavilainen T, Kaitila I. Cementless hip arthroplasty in diastrophic dysplasia. J Arthroplasty. 1992; 7:369-376.
Chiavetta JB, Parvizi J, Shaughnessy WJ, Cabanela ME. Total hip arthroplasty in patients with dwarfism. J Bone Joint Surg Am. 2004; 86:298-304.
Cameron HU, Botsford DJ, Park YS. Influence of the Crowe rating on the outcome of total hip arthroplasty in congenital hip dysplasia. J Arthroplasty. 1996; 11:582-587.

Author

Dr Sekundiak is from the Department of Orthopedic Surgery, University of Nebraska Medical Center, Omaha, Neb.