Overcoming surgical challenges in total hip arthroplasty in dysplastic high-riding hips

Developmental dysplasia of the hip (DDH) is the most common congenital disease of the musculoskeletal system in newborns [1]. If left untreated, limping can occur starting at walking age and premature hip arthritis can occur with severe osteoarthritis detected by the second decade [1, 2]. Severe cases of DDH-related arthritis are more frequently seen in Asia than in the US and Europe, probably because infant screening programs in the US and Europe are more prevalent [3]. The severity of DDH can vary from a subtle dysplasia to mild subluxation to complete dislocation. Due to the highly deformed femur and pelvis, performing total hip arthroplasty (THA) in severe cases of DDH is technically challenging; the complication rate can be high and the survivorship of the implants can be low. In Part 1 of this series, Seung Beom Han from the Department of Orthopedics, Korea University Medical Center, Seoul, South Korea, will discuss what the current practices are in performing THAs in highly deformed, high-riding hips.

Classification of dysplastic hips

Crowe's classification system is the most used system in adult DDH, although the newer system described by Hartofilakidis et al [6] is also a reliable and commonly used system [2–6].
Crowe et al [7], based on their understanding that the degree of difficulty of THAs is associated with the severity of the dislocation and the subsequent distortion of normal bone and soft-tissue anatomy, grouped the dysplastic hips into four types based on the amount of dislocation (subluxation): < 50% in type I, 50–75% in type II, 75–100% in type III, and more than 100% in type IV.

As shown in Figure 1, the amount of deformity in DDH can progress from mild subluxation with a shallow socket (type I) to high dislocation of the hip joint accompanied by an absence of the femoral head and neck and a deformed pelvis (type IV). While a THA in DDH where subluxation is less than 50% has "no greater technical difficulty at surgery than a hip with severe osteoarthritis" [7], THAs in highly deformed DDHs are challenging and require special skills and consideration [2, 7].

Figure 1. Crowe classification: Crowe type I to type IV deformity. X-rays contributed by SB Han.

The Hartofilakidis classification divides DDH into three types: A) dysplasia, B) low dislocation, and C) high dislocation (Table 1).

Table 1. Hartofilakidis classification.

The Crowe classification system for dysplastic hips is reliable, reproducible, and has been shown to be related to the prognosis of patients who underwent THAs [2]; the Hartofilakidis classification and its associated multiple clinical studies offer strategies to manage abnormalities within the types and subtypes. Both are useful and widely used for clinical decision making [4, 5]. The Crowe type IV DDH roughly corresponds to Hartofilakidis type C DDH [8, 9].

Three main challenges in total hip arthroplasty for dysplastic hips

As shown in Figure 1, in advanced cases of DDH deformity, the affected hips can have high dislocation (ie, high-riding hips) and the femoral head is completely out of the true acetabulum. The femoral head usually migrates superiorly and posteriorly to a varying degree—sometimes the femoral head and/or neck can be completely absent, and the pelvis severely distorted. Proximal femoral deformities can include a sclerotic, narrow, and deformed medullary cavity and femoral shaft; the anatomical acetabulum can be small and shallow [3, 8], which has been vividly described by Hartofilakidis as, "…sometimes so narrow that it barely accommodates one fingertip" [10].

A THA in such hips is challenging. Poor bone stock, leg lengthening, altered anatomy, and severe soft-tissue contracture can all put type IV DDH patients at high risk of postoperative complications [3, 11, 12]. To overcome these challenges, the surgeon should conduct careful preoperative planning. Intraoperatively, one should be ready to carry out massive soft-tissue release, take precaution for unintentional fractures, and be prepared for an osteotomy to achieve a stable reduction. In addition, because the patients are usually relatively young, future revision is likely unavoidable. Therefore, one should consider other joint preservation options before venturing into a THA in young patients.

The challenges in performing a THA in high-riding hips can be summarized as:

The acetabulum: identification and preparation of the small and shallow true acetabulum to provide enough coverage and support for the acetabular cup
The femur: management of a range of femoral deformities (narrow and deformed medullary canal, excessive anteversion)
A stable reduction: bringing a high hip dislocation into the true acetabulum without causing soft-tissue injuries, such as nerve palsy

The following sections summarize the strategies in overcoming these challenges.

Identification of the true acetabulum and acetabular component fixation

In performing THA in a DDH hip, it is generally the goal to place the acetabular cup into the true acetabulum. This should yield the best long-term functional results with lower rates of loosening. In addition to being the site with better, relatively sufficient bone stock, the true acetabulum is also the anatomical hip center that provides optimal biomechanics [3, 7, 13, 14]. In patients with type IV DDH, the temptation is to succumb to choosing a high hip center reconstruction, which would mean an increased risk of postoperative dislocation needing revision surgery [11]. Han reminds us that, "in placing the acetabular cup at the level of the true acetabulum, one can achieve better abductor muscle strength, decrease the joint reaction force, and equalize the limb length".

In high-riding hips, the femoral head can articulate with the false acetabulum at varying levels on the ilium. Since most of the DDH hips will have some degree of bone loss of the superior acetabulum, regions more proximal to the true acetabulum usually provide better coverage. However, because the ilium becomes significantly thinner proximal to the acetabulum, such an advantage eventually disappears [3]. The greatest available amount of bone stock is at or near the level of the true acetabulum. In this regard, Crowe IV acetabular defects are somewhat easier to manage than those in Crowe type III hips because the femoral heads are not located in the true acetabula, so the true acetabula are likely to have minimal erosion—although they are often very small and atrophic [3, 7].

Identifying the true acetabulum can be difficult at times. It can be done by following the inferior joint capsule at the distal cotyloid notch or transverse acetabular ligament [3, 11, 14]. Once the true acetabulum is identified, the scar tissue and osteophytes should be removed before reaming. The reaming of the true acetabulum should be performed with care, starting with a small diameter reamer until the medial wall is reached with bleeding cancellous bone [9, 10]. The reaming can then be expanded toward the anterior or posterior column to maximize cup coverage, but care should be taken to avoid violating the important posterior wall and column. If proximal support is still lacking and the coverage is still less than 70%, an augmentation with bone graft using the cancellous bone chips from the resected femoral head can be done [3, 7, 11]. Additional cup support techniques, such as dome screws, can also help improve the cup stability [7, 9, 11]. A slight, controlled medializing of the cup toward the medial wall (cotyloplasty) may provide additional increase in the cup coverage, but overzealous medialization can result in cup migration, loosening, and osteolysis [6, 12].

With the increase of DDH deformities, anteversion in both the acetabulum and femur becomes more extreme. For example, in Crowe type IV hips, a mean acetabular anteversion has been reported to be 33.28° ± 5.98° as compared with 20.46° ± 7.48° in normal hips [15]. As reviewed by Shi et al [15], a useful method to prevent postoperative dislocation is to keep the combined acetabular and femoral anteversion under 55° (20–25° for the cup and 15–20° for the stem) (Figures 2 and 3) [9, 11]. In cases where an osteotomy (see the later section on subtrochanteric osteotomy [STO]) is required, one can set the cup anteversion according to the native acetabular anteversion to enable preferable cup coverage; after the STO, the femur can then be rotated to maintain appropriate combined anteversion [9, 16].

Figure 2. Measuring the anteversion of a right acetabulum on a CT scan. Line a is the midline of the transverse section of the body. Line c connects the anterior and posterior margins of the right acetabulum. Line b is parallel to line a. The angle formed by b and c is the anteversion of the right acetabulum [15]. Figure reproduced under the CCBY-NC 4.0 terms https://creativecommons.org/licenses/by-nc/4.0/.

Figure 3: Measuring the femoral anteversion on a CT scan. CT scans of the same side hip (a) and knee (b), taken at the same time and with no change in leg position. (a) Line ab is at the middle of the femoral neck and crosses through the center the femoral head. Line bc is the base line and line b'c' is parallel to it. (b) Line b'd is the posterior condylar line of the femur. The femoral anteversion = angle 1 + angle 2. If the knee has an inward rotational angle according to the base line b'c', its value is negative; if the knee has an outward rotational angle, the value of angle 2 is positive [15]. Figure reproduced under the CC BY-NC 4.0 terms (https://creativecommons.org/licenses/by-nc/4.0/).

Because the true acetabula are usually underdeveloped and are shallow and small, even with the techniques described above, they may not be large enough to still accept a normal-sized cup. In this case, extra-small cups and femoral components are necessary. How small should these implants be? Han tells us, "In handling highly dislocated hips, we have developed the strategy of deep reaming and use of small-sized components. For example, a combination of cup size of 42 mm diameter (or smaller) and a cobalt-chrome head of 22 mm have worked well in our hands."

Femoral deformity

Femoral deformity in advanced cases of DDH can include a small or absent femoral head, anteverted femoral head and neck with posterior migration of the greater trochanter, and narrow, straight, distorted intramedullary cavity that is wider in the anteroposterior but narrower in the mediolateral dimension [7]. Because of these deformities, careful preoperative templating is critical and will help with selecting the most suitable implants [15].

Recent advancement in implant technology has greatly facilitated THAs in DDH hips. Although in cases where the femoral anteversion is < 25°, the traditional monoblock stems could work [11] in treating highly deformed DDH hips, the newer of cementless modular stems are superior. Modular stems make it easier to adjust femoral offset, deal with increased femoral anteversion, and a mismatch between a larger metaphysis and a narrow diaphysis [9, 11]. The S-ROM modular hip system is currently the favorite stem in conjunction with STO because its fluted stem provides the needed additional torsional diaphyseal stability [3, 13]. Han tells us, "other stems, such as the Wagner type fluted cone stems, provide easy control of anteversion and excellent rotational stability especially in case of STO". (For a review of the modular stems, see Part 3 of this series.)

The improvement in the bearing surface is another important innovation. Due to the young age of many DDH patients, the wear of bearing surfaces from conventional polyethylene means a shortened lifetime of a THA and potentially increased number of revisions [3, 15, 17]. Park comments, "metal-on-metal bearings may have caused inflammatory reaction in some patients and ceramic-on-ceramic is good in terms of wear and reaction, but the concern of breakage in 22 mm ceramic heads has limited its use. A large amount of published data on highly cross-linked polyethylene points to a potential long-term success with little wear and osteolysis of this material."

Bringing the high dislocation femoral head down to the true acetabulum

In Crowe type IV or Hartofilakidis type C DDH hips, bringing the femoral head down to the level of the true acetabulum means a large amount of leg lengthening, ie, proximal migration of the femoral head. Sometimes even an extensive soft-tissue release may not be enough and nerve palsy can happen. Although nerve injuries can have different causes, many surgeons consider excessive leg lengthening to be the main cause [11]. For this reason, preoperative evaluation of x-rays or CT scans should be done to determine if the amount of leg lengthening would exceed 3–4 cm; if so, a femoral shortening by osteotomy should be planned [8, 11].

Currently, STO is the most popular technique for femoral shortening in Crowe type IV hip dysplasia. In addition to femoral shortening, STO also provides the chance to correct excessive anteversion [3, 8, 11]. An STO can be performed in several geometries (Figure 4), including transverse, oblique, double chevron (V-shaped), step-cut (Z-shaped), and sigmoid (not shown in Figure 4) [11]. The transverse cut is generally favored due to its simplicity and ease of rotational correction; the drawback of this method is the potentially higher chance of rotational instability leading to delayed bony union. Although other types of osteotomies have promised better rotational stability and more bony contact to support early bone union, so far no superior clinical results have been demonstrated [18, 19].

Figure 4. Types of cuts for a subtrochanteric osteotomy. Illustration adapted from the illustration provided by SB Han.

Although STO has shown good results in some hands, others have seen increased complications such as nonunion, after STO [11, 13]. To circumvent the shortcomings of an STO, some have proposed alternative techniques to prevent excessive leg lengthening. Techniques such as distraction with an external fixator [11], proximal femoral reconstruction [8], distal femoral shortening osteotomy [21], and iliofemoral distraction prior to THA [20] can also provide satisfactory outcomes, but these may have other drawbacks that limit their use [11].

4. The final femoral components can now be inserted. A cerclage wire should be placed around the proximal and the distal sites of osteotomy before implant insertion to avoid intraoperative fractures (Figure 8). The two femoral fragments should be rotated to achieve the correct anteversion and good alignment. Check to make sure that a tight press fit with good rotational stability (both proximally and distally) is obtained. The femoral fragments obtained from the previous step can be fixed with two cerclage wires and used as cortical autograft.

Figure 8. Final insertion of the stem and adjusting the alignment. The final construct is illustrated in the right-hand panel. Illustration adapted from the illustration provided by SB Han.

3. Gentle traction can now be applied to the leg to determine the amount of overlap, ie, the amount of femur to be shortened (Figure 7). The tension of the sciatic nerve can be monitored by palpation of the nerve to avoid excessive stretching. Conventional wisdom is that a limb lengthening of more than about 4 cm may increase the chance of nerve injury and nerve palsy [3, 9, 11]. The second transverse osteotomy is now performed and the resected femoral fragment with the attached soft tissue can be split and later used as cortical strut autograft.

Figure 7. Determining the amount of femoral shortening. After the femoral head is brought down to the level of the true acetabulum, the amount of overlap is the amount of femoral shortening. Illustration adapted from the illustration provided by SB Han.

Outcomes

Published results for THA in Crowe type III or IV DDH have so far been encouraging, although long-term results are still not abundant. Table 2 summarizes several studies on the mid- to long-term (minimum follow-up of 5 years) outcomes of cementless THA in Crowe type IV hips, all performed with concurrent STO. Three studies reported over 90% of survivorship at 10 years [9, 21, 22], one study reported 11.7% revision rate at the mean follow-up of 12.9 years (5.2–16.8), but only two revisions were due to aseptic loosening [23], and one study where the survivorship was not analyzed but no revision was recorded [24].

Aside from good clinical outcomes as demonstrated by various outcome instruments (multiple scores may have been assessed, but only the Harris hip scores are captured in Table 2), improvement in pain, limping, gait, etc have also been demonstrated [22–25]. One common limitation of all studies presented here is that all were retrospective studies with small sample sizes.

Table2. Mid- to long-term outcomes of cementless THA in Crowe type IV hips. Cl, confidenceinternal; CoC, ceramic on ceramic; MoP, metal on polyethylene.

Conclusion

Even with implants that provide surgeons with more flexibility in highly deformed hips, THA in high dislocation is still challenging. An essential element of proper preoperative planning should include the planning of multiple implant options. Reconstruction of acetabulum to the true acetabulum is ideal and subtrochanteric shortening osteotomy can help achieve a stable reduction and prevent nerve palsy. Modular or fluted cone type stems are greatly helpful in controlling the anteversion and providing diaphyseal rotational stability.

Performing a subtrochanteric osteotomy

We ask Han to share how he performs an STO, and he advises the following procedures:

1. Cut the femoral neck about 1 cm proximal to the lesser trochanter and perform the intramedullary reaming and broaching before the subtrochanteric osteotomy. The reaming should go distally to the intended osteotomy site (see the right-hand panel in Figure 5).

Figure 5. Preparing the proximal femur for a subtrochanteric osteotomy. The femoral neck is resected and a trial stem is used to estimate the amount of reaming needed. Illustration adapted from the illustration provided by SB Han.

2. The first transverse subtrochanteric osteotomy is performed distal to the metaphyseal flare of the implant (top yellow line in Figure 6). Insert the trial femoral stem and adjust the anteversion to the normal range, then perform a trial hip reduction.

Figure 6. Preoperative planning. Yellow lines indicate sites of osteotomy. X-ray provided by SB Han.

Part 2 | Childhood infection sequelae

Part 3 | Patients with skeletal dysplasia

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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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