Total hip arthroplasty in patients with skeletal dysplasia

Skeletal dysplasia, also called osteochondrodysplasias, is different from developmental dysplasia of the hips (DDH), which has been introduced in Part 1 of this series. While DDH normally causes isolated hip joint deformity as a primary condition—with possible involvement of other joints as secondary problems—patients with skeletal dysplasia, a genetic disorder, are usually short in stature and have primary deformities in multiple joints, such as in hips, knees, and spine. It is not unusual for patients with severe forms of skeletal dysplasia to require total hip arthroplasty (THA) at a young age for pain relief and function improvement. Aside from whether good outcomes can follow a total hip arthroplasty in these severely deformed hips, maximizing the survivorship of such procedures is also a serious concern because these patients will likely have one or more revision surgeries in their lifetime. In this part, Seung-Jae Lim from the Department of Orthopedic Surgery, Samsung Medical Center, Seoul, South Korea, shares his knowledge with us on this topic.

Skeletal dysplasia: a term covering many hereditary conditions

Skeletal dysplasia (osteochondrodysplasia) is an umbrella term for more than 350 hereditary conditions of abnormal cartilage and bone development; patients suffering from skeletal dysplasia are afflicted with varying degrees of short stature [1, 2]. Skeletal dysplasia is genetically heterogenous and can be inherited as autosomal dominant, autosomal recessive, X-linked recessive, X-linked dominant disorders, or other rarer modes of transmission. Although each skeletal dysplasia is relatively rare, collectively, different forms of skeletal dysplasia have an incidence of about 1 in 5,000 births [1]. The classification of these skeletal disorders based on radiographic abnormality in the different parts of the long bones, ie, epiphysis, metaphysis, and diaphysis, are most widely used. Further, spondyloepiphyseal, spondylometaphyseal, and spondyloepimetaphyseal dysplasias describe disorders with spinal involvement.

Multiple epiphyseal dysplasia (MED) and spondyloepiphyseal dysplasia (SED) are two common forms of skeletal dysplasia [1, 3]. They are associated with different degrees of abnormalities of the skeleton, resulting in disproportionate short stature (ie, dwarfism) and orthopedic complications. A diagnosis of skeletal dysplasia is usually based on clinical, radiographic, and molecular findings [1]. Lim tells us, unlike high hip dislocation and infection sequelae, which are more prevalent in Asia and developing countries, the incidence of skeletal dysplasia is similar in Asia/developing countries and the US/Europe.

In patients suffering from skeletal dysplasia, arrested epiphyseal development causes delayed endochondral ossification that cannot support the cartilaginous surface, leading to damages in the loaded cartilage surface. Cartilage of poor quality, altered biomechanics, and soft-tissue changes all lead to early-onset osteoarthritis [4–7]. Due to heterogeneous genetic penetration and expression, the manifestation of the diseases can vary; even in the same affected individual, the changes in the epiphysis can be mild in one site and severe in another [5]. Since the lower limbs bear much of the normal physiological load, joints of the lower limbs are especially susceptible to arthritic changes in patients with skeletal dysplasia, and the hip joints are invariably affected, leading to the need for THA at a relatively young age [5, 7, 8]. Total hip arthroplasty can be indicated in some adult patients with substantial pain and disability from end-stage osteoarthritis of the hip joint. A THA in these deformed hips with soft-tissue contracture, however, is surgically challenging and may have increased complications [9, 10].

Radiographic features

Patients with dwarfism often present with anatomical features such as a broad and short femoral neck, accompanied by flattened femoral head with poor acetabular coverage, and coxa vara. The deformed femur is often widened proximally but narrowed distally with an anterior bowing of the shaft [2, 4]. When the spine is involved, aside from the usual involvement of complex hip deformities, vertebrae are usually observed to be irregular and flattened [3].

Figure 1 demonstrates the typical radiographic features of an MED case with no spinal involvement (normal vertebrae). Typical features listed above, ie, large, flattened femoral heads, shortened femoral neck resulting in decreased horizontal offset, and shallow acetabulum with poor coverage are easily seen. Such a configuration is biomechanically unstable and leads to early onset of arthritis. Notice also the osteopenia bones, which were caused by long years of arthritis and pain that led to disuse and reduced bone stock. This reduced bone stock in combination with deformed proximal femur may increase the risk of periprosthetic fractures intra- or postoperatively.

Figure 1. Radiographic features of a dysplastic hip with multiple epiphyseal dysplasia. Images of a 42-year-old, 153 cm tall man with severe arthritis in the hip (a), dysplastic knees (b), and dysplastic ankles (b). Since the vertebrae are normal, the patient is diagnosed with multiple epiphyseal dysplasia and not spondyloepiphyseal dysplasia. X-rays contributed by Seung-Jae Lim.

Surgical challenges of total hip arthroplasty in skeletal dysplastic patients

The surgical challenges of performing THAs in patients with skeletal dysplasia come largely from the deformities, which pose challenges in more than one way. We asked Lim to share some tips with us concerning what one must pay attention to. Lim says, "the most important tip is that careful preoperative planning is critical in ensuring successful THA in these patients". He also reminds our readers that, "common types of prostheses are often inadequate for patients with short stature even if different component sizes and modularity are provided. The selection of implants of suitable sizes and lengths must be carefully planned based on patients' preoperative radiographs." Aside from that, there are three areas of risks one should watch out for: intraoperative periprosthetic fractures, nerve damages, and increased chance of infection. These are elaborated below:

Periprosthetic fractures can occur either due to poor bone stock or the atypical femoral morphology; this can be handled by using cerclage wires either preventatively or after fractures have occurred (Figure 2).
Massive soft-tissue release may help avoid nerve damages and careful reaming may help avoid damaging the shallow acetabulum. Because of the short femoral neck and the short stature of this patient population, the space may be limited for surgeons to reach the optimal position and orientation for a proper implant insertion. Extensive soft-tissue release can make the implant insertion easier by creating more space, but one still must be careful when stretching and lengthening the offset, so as not to cause iatrogenic damages.
Because of the complexity of THA in these deformed hips, surgeries will likely take more time. Longer operation time is one factor that increases the risk of infections [6, 8]. Prophylactic antibiotic usage may help prevent postoperative infections [11].

Before the modular stems were commonly available, conventional types of prostheses were often inadequate for patients with short stature [2, 4]. Offset and neck lengths of conventional prostheses were too excessive and the stems too long to accommodate the femoral bowing in small patients, leading to, for example, the reported off-label use of transected stems [2]. The availability of femoral stems with small diameters and the invention of modular stems (see the next section) have eased some aspects of the surgical challenges, but such surgeries are still very challenging. A recent study showed that patients with skeletal dysplasia are more likely to undergo THAs in a teaching hospital than patients without skeletal dysplasia, presumably reflecting the technical challenges of these surgeries [6].

Figure 2. The 5-year follow-up x-ray after bilateral modular cementless total hip arthroplasties. The intraoperative calcar crack in the right femur was managed by cerclage wiring. X-ray contributed by Seung-Jae Lim.

Modular stems: why?

Instead of using the original Chanley’s monoblock stem that contains a body, neck, and head all in one unit, almost all modern femoral stems now have proximal modularity with modular heads [12]. For this reason, the term “monoblock” is sometimes used to refer to an implant with a monolithic stem that contains a body and a neck, to be coupled with a standard modular head (Figure 3) [13].

Additional proximal modularity includes modular shoulders, necks, collars, and sleeves [12]. Aside from modular proximal designs, modularity can also be applied as mid-stem and distal modularity. In a complex revision THA with poor proximal bone stock, mid-stem modularity is particularly useful in solving the mismatch between proximal and distal femoral anatomy—as long as a stable fixation of the stem in the distal diaphysis is secured. Distal modularity was initially developed mainly for revisions to increase distal fixation and reduce thigh pain, but it has failed to show clinical relevance [12].

The performance of different modular components of a stem has been mixed. While a proximal modular neck has been proven problematic (see the next section) and has largely been withdrawn from the market, modular proximal sleeves have been hailed as the greatest achievement in the history of modular stems (Figure 4) [12].

The proximal modular sleeve was invented to maximize the contact between femoral bone and the implant and has been applied to, for example, the S-ROM stem. The porous-coated, modular sleeve with steps in the S-ROM stem provides a tight fixation between the proximal metaphysis and the stem to ensure proximal stability. The proximal sleeves are available in different heights and widths, so that intraoperatively, a surgeon can choose a sleeve that best fits the patient's metaphyseal morphology. In combination with different neck styles, surgeons can modify the deformity and achieve optimal medial and vertical offsets. Along with the fluted distal stem design that provides good rotational stability, the S-ROM stem has been used continuously without major changes since 1985, and good results have been reported in various deformed hips, such as skeletal dysplasia and developmental dysplasia of the hip (see the later section on clinical outcomes and Part 1 of this series) [12].

Figure 3. Monoblock (left) and dual modular femoral stems (right). Both stems could be fitted with separate heads of a different material, size, and bore length. The dual modular stem has been explanted 14 years after primary implantation at revision due to aseptic loosening of the acetabular component and massive proximal femoral bone loss [13]. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

As reviewed by Park et al [12] in 2018, by combining the available sleeves, stems, and heads at that time, 10,398 possible combinations could be created. If one considers in addition the independent rotation (or version) of stems, there are almost infinite combinations. This abundance of variations is particularly important for surgeons facing a challenging THA with multiple deformities in both the acetabular and the femoral side. Modularity in hip stems allows surgeons to adjust offset, version, rotation center, and limb length independently from one another, both during preoperative planning and in the operating room.

Controversies over modular stems: the neck-stem junction

Although modularity makes it easier for surgeons to adapt the prostheses to an individual's anatomy in performing THAs, with increasing numbers of femoral component connections, the chance of complications such as component corrosion and breakage and soft-tissue damages caused by wear debris from the implants also increases [13, 14]. Dual modular stems with a head-neck junction and a neck-stem junction (as opposed to "monoblock" stems that have only one, ie, the head-neck junction), have been particularly controversial, because modular neck fractures usually occur at the additional neck-stem junction level.

Complications after THAs with modular stems can be divided into two categories based on the causes: mechanical and inflammatory. Mechanical complications are caused by micromotions generated between two connecting components during daily activities. These micromotions wear at the materials and presumably contribute to the reported sudden fractures at the neck-stem junction, which has been reported to occur as early as at 2 years after implantation [13]. Many guidelines and technical advice have been published with the aim to help minimize micromotions; some are supported by better evidence than others. Such advice and guidelines include: not to use large heads more than 36 mm in diameter that may cause junctional corrosion, avoid modularity in obese and very active patients, clean and dry the components carefully before assembly, and apply appropriate impaction force when assembling the components (for details, see Mertl et al [14]).

Aside from causing breakage of the implants, micromotions and mechanical wearing also lead to the releasing of metal debris and metal ions from implants into the periarticular environment, causing adverse inflammatory tissue reaction and pseudotumors [13–15]. Although micromotion and implant-specific mechanical factors can influence implant corrosion, other factors such as the pH of the joint fluid and the composition of the implant materials can also affect implant corrosion [14]. As reviewed by Zajc et al [13], wear particles of different metal ions have distinctly different distribution patterns and can affect biological and physiological pathways differently. In severe cases of inflammatory response and tissue destruction, well-fixed stems may need to be extracted and the THA revised. The revision, unfortunately, frequently needs extended trochanteric osteotomy and leads to increased morbidity.

According to the Australian Orthopaedic Association National Joint Replacement Registry of 2020, the revision rate of femoral stem with modular necks was almost twice as high as that of fixed neck stems, especially if the titanium stem/cobalt chrome neck (as opposed to titanium stem/titanium neck) combination was used [16]. At 15 years, the cumulative incidence of metal-related pathology was 3.9% in titanium/cobalt chromium prostheses compared to 0.1% for titanium/titanium ones [15]. Currently, many have advocated for the banning of titanium/cobalt chromium in favor of titanium-titanium connections [14, 15].

Based on the current knowledge about the modular necks, the concerns of mechanical and inflammatory complications, and the lack of evidence of clinical advantages, it has been recommended by many that modular necks should be used only in difficult cases, for example, in dysplastic hips or severely deformed femurs [12–15].

Current clinical outcomes

Due to the rarity of this condition, literature evidence on the outcomes of THA in patients with skeletal dysplasia is very scarce and the reported outcomes are mostly, if not all, from retrospective studies with small numbers of patients. Inevitably, these results do not always agree with one another. For example, while Lim et al [11] reported excellent results with no hip requiring revision because of aseptic loosening in 23 hips treated with cementless modular stems followed up to a mean of 4.8 years (range, 2–8 years) and Guenther et al [4] reported an overall implant-associated survival rate of 99.1% at 1 year and 92.6% at 5 years [4]—both with > 40 points gain in Harris hip score, an older study reported an implant failure rate of 29% (18 in 62 hips) [9]. It may be that the differences reflect the difference in older studies (using prostheses from older technology) versus newer ones [10], but the differences could also be due to the evaluation of multiple forms of dwarfism and skeletal dysplasia, heterogeneity in the severity of the deformities, usage of implants of different materials and designs, or other factors.
To demonstrate their point that newer materials may influence THA outcomes in skeletal dysplasia, Wyles et al [10] analyzed the implant survivorship in patients with SED according to the bearing surface materials of the implants they received. The Kaplan-Meier survivorship analysis showed that the 5- and 10-year survivorship of implants with highly cross-linked polyethylene bearing surface was 96% as compared to the survivorship of implants with conventional polyethylene bearing surface of 96% at 5 years and 82% at 10 years. The authors concluded that modern implants with highly cross-linked polyethylene bearing surface and modular (or custom) design can be reliable options for patients with SED suffering from symptomatic coxarthrosis.

It is worthwhile to mention that Modi et al [17] performed a matched cohort study to compare the survivorship of THAs in patients with short stature versus the normal controls, followed up to 10 years. The matching was based on age, gender, year of surgery, and Charlson comorbidities [17]. Modi et al reported the survivorship of THA in patients with dwarfism at 2, 5, and 10 years was not statistically different. Another recent matched cohort study analyzing the results of primary THA performed from 2012–2017 in patients of short stature versus the control cohort also found that the postoperative complication rates showed no statistical differences, although the operative time was longer for patients with short stature [8].

Conclusion

Some older studies have reported relatively high rates of complications, and this continues to be a concern in the patient population with skeletal dysplasia. Thanks to the modern innovation in both implant materials and designs, it is possible to achieve good midterm (up to 5–10 years) results and to relieve pain and improve functions for these patients. It should not be forgotten, however, that even with modern technology, THAs in these patients with severe anatomical deformities are still challenging. A thorough understanding of the bone and soft-tissue deficiencies that one may encounter intraoperatively, being knowledgeable in the strategies for handling them, and meticulous preoperative planning are the prerequisites for surgeons who want to take on the challenge of performing THAs in severely deformed hips.

Figure 4. Implants with modular proximal sleeves. (a) Modified Sivash stem. (b) S-ROM modular stem (Courtesy of DePuy Orthopaedics, Warsaw, IN, USA) [12]. This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0 ), which permits unrestricted noncommercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Part 1 | Dysplastic high-riding hips

Part 2 | Childhood infection sequelae

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Contributing experts

This series of articles was created with the support of the following specialists (in alphabetical order):

Seung Beom Han

Department of Orthopedics
Korea University Medical Center
Seoul, South Korea

Seung-Jae Lim

Department of Orthopedic Surgery
Samsung Medical Center
Seoul, South Korea

Youn-Soo Park

AO Recon Education Forum
Department of Orthopedic Surgery
Samsung Medical Center
Seoul, South Korea

This issue was written by Maio Chen, AO Innovation Translation Center, Clinical Science, Switzerland.

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