Uncertainty, lack of data complicate use of short stem hip implants

Hip implants featuring a shortened femoral stem are designed to preserve bone stock during total hip arthroplasty while offering a more physiologic replication of normal human anatomy compared with longer stem prostheses. Nearly a decade after their release, however, there is a growing sense among orthopaedic surgeons that, despite a strong theoretical basis for their use, clinical data may be as of yet lacking on whether short stem hip implants fulfill their potential. Questions remain about what types of patients benefit from their use, the optimal surgical technique for their insertion and whether they replace or supplement the current total hip arthroplasty implant offerings.

Short stem total hip arthroplasty (THA) has generated significant interest in surgical centers across Europe. And, although consensus on short stem hip arthroplasty remains elusive, some experts believe this option, which tends to foster less invasive surgery, has significant clinical benefit.

“The introduction of the short stem implant has extended the range and indications of minimally invasive hip arthroplasty,” Dieter C. Wirtz, MD, of University Hospital, Bonn, Germany, told Orthopaedics Today Europe. “When used within their indications and implanted correctly, the short stem implants are comparable in their risks of instability, fracture or failure to a conventional uncemented hip prosthesis.”

In general, short stem implants are designed to require less resection of the upper femur and/or less reaming of the femoral shaft. This serves a dual purpose of facilitating future revision while providing a postoperative state closely mimicking the originally functioning hip.

Dieter C. Wirtz, MD, of University Hospital, Bonn, Germany, cited the benefits of short stem hip arthroplasty as a minimally invasive surgical technique and possibly better results if a revision procedure is later indicated. Image: Claudia Siebenhuener/Universitätsklinikum Bonn

“Some believe that a more physiological result is obtained as the bone and proximal femur retains its normal morphology,” Orthopaedics Today Europe Editorial Board member Professor Fares S. Haddad, BSc, MCh (Orth), FRCS (Orth), FFSEM, of University College London, said.

“Short stem prostheses mimic the physiological situation of force transduction from the hip joint into the femoral bone, where the metaphysis will take up the major part of the loading. This will prevent bone loss by stress shielding in this area,” Wirtz said.

The specific preservation of femoral neck and a properly designed short stem implant allow correct stress distribution and avoid stress, according to the most recent biomechanical stress distribution theories by Joseph Fetto, MD, and Peter S. Walker, PhD. This design consideration, according to Francesco S. Santori, MD, accounts for the normal physiologic state of load transfer throughout the hip joint.

“This load must be distributed on the medial and lateral anterior and posterior walls of the proximal femur,” he said.

According to Santori, a member of the Orthopaedics Today Europe Editorial Board, short stem implants generally rely on metaphyseal stem placement, but some do not utilize any support on the metaphyseal bone, which may make them more prone to failure or loosening if bone stock is of poor quality. THA implants fixed to the proximal femur, which are becoming more popular across Europe, may help correct this.

The category of implants employing lateral fixation, such as the Proxima (DePuy, a Johnson & Johnson Company), which Santori helped design, preserves the anterior and posterior wall of the femoral neck, which, along with a larger diameter than most short stem prostheses, may help control for migration while diminishing the effect of torsional forces within the hip joint. With this type of implant the indications are the same as a conventional cementless stemmed implant, Santori said.

The overarching rationale for short stem implants is the preservation of bone within the hip area. According to Luigi Zagra, MD, of Istituto Ortopedico Galeazzi, Milan, Italy, “they were created to preserve the neck of the femur,” which may be important for “younger patients who may need further surgeries. But many times the bone preserved is very small and the revision of a stable short stem is not easy. Moreover, long-term results have shown if a standard stem is implanted in the correct indication and femoral shape, stress shielding is avoided.”

Younger, more active patients

Exactly what constitutes a “young” patient remains ill-defined. Some view the label in terms of age, while others interchange the word “young” with “active,” extending indicated use to populations that may be older, but who place high demand on an active lifestyle.

“We are using the short stem THA in active patients below the biological age of 65 in males and 60 in females, with a high demand to their joint function and good bone quality,” Wirtz said. “In combination with a ceramic-on-ceramic bearing to minimize wear and attrition, we mainly see the advantages of the short stem implant in the minimal invasive operative technique and the possible benefit for subsequent revisions.”

However, it is thought that using such implants in younger, more active patients may preselect for a population with greater odds for revision. In turn, the increased chance of future revision inherent to younger, more active populations may artificially skew reported revision rates, creating a perception that short stems are more prone to failure.

“With the lower age at the time of primary arthroplasty, the risk of a subsequent need for revision in the years to come rises,” Wirtz said.

Typically, revision has been required for instability, fracture or failure of the original implant.

“Personally, I think there is a greater risk since the implantation is technically more demanding. But, it is too early to answer the question,” Gerold Labek, MD, of Innsbruck Medical University, Innsbruck, Austria, told Orthopaedics Today Europe.

While there are no definitive answers on whether short stem implants are in fact more prone to fracture, instability or other types of implant failure, neither is there sufficient data on their performance. The risk-benefit analysis is decidedly incomplete, and it may well turn out that some inherent shortcomings are acceptable if more positive effects can be proven, Labek said.

Then again, short stem implants may not even address the core issue associated with implant failure, according to Zagra, who noted failure of the hip cup is three times more likely than stem failure to spur a need for revision. Even if shorter stems perform as expected, their impact on the need for future revision may be marginal at best.

“For the short stem, the really critical period is the short term.” — Luigi Zagra, MD

Long-term data

Because they are relatively new to the orthopaedic marketplace, there is limited clinical data on short stem THA. However, Santori said long-term data do exist for some models, namely the Proxima and the CFP (Link), which he said were both introduced about 15 years ago.

Santori has performed more than 500 procedures since 1995 using the Proxima implant or a prototype similar in design and function. In that time, only two cases required revision due to septic loosening with no cases of aseptic loosening. Neither has instability been a significant issue, as less than 1% of cases suffered a dislocation, which he said may be attributable to his use of a 36-mm head whenever possible.

However, according to Zagra, outside the setting of closely monitored, industry- or designer-conducted studies, there is not much data to fairly judge whether the theoretical benefits of this well-conceived technology have been achieved. That puts the onus on surgeons to extrapolate from what has been learned so far from registries, case series and small clinical trials.

For instance, Zagra said the distribution of positive and negative outcomes may suggest a learning curve exists relative to new technology integration as more implant failures and revisions were noted in the periods just after the release of a given stem, followed by, in most cases, a downward trend in number of cases. Presumably, this declining incidence of negative outcomes over time represents efforts by innovators to refine the procedure and technology to increase their success. This would be understandable, he added, given the complexity of hip replacement in general, and specifically the accuracy required for placing short stem implants.

To avoid the potential for stress shielding, it is widely recognized that metaphyseal placement of a short femoral stem must be accurate and precise so as to facilitate transfer of load down the femoral shaft. Any malpositioning or a suboptimal surgical technique could transfer load to the outside of the femur, which may produce osteopenia. An increase in lateral weight transfer may also increase the risk of the head slipping from the acetabular cup, Haddad explained.

The smaller size of the implant may also present unique operative issues, namely with decreased surface area for fixation. According to Haddad, “there is a greater risk of failure by aseptic loosening.”

“In terms of gaining fixation, some of these require a very tight fit and as a result there is a slightly increased risk of fracture,” he said.

Defining indications

One finding in the current literature is that the predominance of failures requiring revision associated with short stem implants occur in the short-term postoperative period, suggesting that achieving immediate stability is critical to success.

“There is a higher rate of revision in the immediate postop period with short stems,” said Zagra, an Orthopaedics Today Europe Editorial Board member. “For the short stem, the really critical period is the short term.”

Clinical evidence thus far suggests if stability is achieved in the immediate postoperative period, performance of short stems appears to be on par with conventional stems. However, achieving stability, Zagra said, depends on intraoperative alignment and on selection of appropriate surgical candidates.

But, not all surgeons agree on indications for use. Labek said he does not use short stem devices in his practice, but noted they may be used virtually interchangeably with conventional stems if all other patient parameters are equal (namely that there is sufficient bone stock to support the implant and subsequent load transfer).

Santori agrees short stems can be used in essentially the same kind of patient as a conventional stem given there is sufficient bone stock. He uses short stems in 93% to 95% of cases, and infrequently chooses cemented fixation.

In fact, Santori said, short stem hip arthroplasty may eventually replace the practice of resurfacing, which has become a popular option for younger patients seeking a less invasive technique (ie, shorter recovery and quicker resumption of activities). Studies have noted significant femoral head necrosis following some failed resurfacings. Although he has limited experience with that technique, having performed only about 30 such operations, Santori said he has confirmed this finding in histological studies of failed resurfacing cases.

“It has been argued that patients are happier with resurfacing because there is less discomfort, which is present after conventional cementless implantation. With the short stem, the sensation of the patient is similar to that of resurfacing, but without the associated complication of implant failure,” he said.

Short stem THA has been studied in a wide variety of patients, but many surgeons believe young, active patients will benefit most from its use. Regardless of age or activity level, the patient’s anatomical condition is a key to the ultimate success of implantation.

“In patients with poor bone quality, severe osteoporosis or a femoral head necrosis reaching into the metaphyseal region, the indications are limited,” Wirtz said. “A thorough patient assessment, with special regard to the metaphyseal bone quality and biomechanical situation of the affected joint, as well as thorough surgical planning, precise osteotomy and correct positioning of the implant, helps in maximizing long-term stability of the implant.”

To others, the lack of long-term follow up complicates how readily short stems are being used, largely because there remains sufficient uncertainty about not only how to use these devices, but also for whom they are indicated.

“Like many innovations, some [devices] have performed extremely well, whereas others unfortunately have seen early failures and a narrowing of their indications,” Haddad said. “This is an area where it is critical that the best standards for introduction of innovation by prospective randomized studies and careful registry data collection are maintained, such that major mistakes do not occur.” – by Bryan Bechtel

References:

• Fetto JF, Austin KS. A missing link in the evolution of THR: "discovery" of the lateral femur. Orthopedics. 1994;17:347-351.
• Fetto JF, Bettinger P, Austin KS. Re-examination of hip biomechanics during unilateral stance. Am J Orthop. 1995;8:605-612.
• Fetto J, Leali A, Moroz A. Evolution of the Koch model of the biomechanics of the hip: clinical perspective. J Orthop Sci. 2002; 7:724-730.
• Kim YH, Kim JS, Park JW, Joo JH. Total hip replacement with a short metaphyseal-fitting anatomical cementless femoral component in patients aged 70 years or older. J Bone Joint Surg Br. 2011;93(5):587-592.
• Kim, Y-H, Choi, YW, and Kim, J-S. Comparison of bone mineral density changes around short, metaphyseal-fitting, and conventional cementless anatomical femoral components. J Arthroplasty.2010 Dec 3. [Epub ahead of print]
• Kim, Y-H, Kim, J-S, Joo, J-H, and Park, J-W. A prospective short-term outcome study of a short metaphyseal fitting total hip arthroplasty. J Arthroplasty. 2011; Mar 22. [Epub ahead of print]
• Kim YH. Is diaphyseal stem fixation necessary for primary total hip arthroplasty in patients with osteoporotic bone (Class C bone)? J Bone Joint Surg Am. In Press.
• Santori FS, Manili M, Fredella N, et al. Ultra short stems with proximal load transfer: Clinical and radiographic results at five year follow-up. Hip Int. 2006;16(suppl 3):S31-39.
• Santori FS, Santori N. Mid-term results of a custom-made short proximal loading femoral component. JBJS Br. 2010; 92(9):1231-1237.
• Santori N, Albanese CV, Learmonth ID, Santori FS. Bone preservation with a conservative metaphyseal loading implant. Hip Int. 2006;16 Suppl 3:16-21.
• Walker PS, Culligan S, Hua J, et al. The effect of the lateral flare feature on uncemented hip stems. Hip Int. 1999;9:71-80.

• Fares S. Haddad, BSc, MCh (Orth), FRCS (Orth), FFSEM, can be reached at Institute of Sport, Exercise & Health, Division of Surgery & Interventional Science, 4th floor, 74 Huntley St., London, UK WC1E 6AU; +44-207-935-6083; email: fares.haddad@ucl.ac.uk.
• Gerold Labek, MD, can be reached at Anichstrasse 35, Innsbruck A-6020, Austria; +43-512-504-81600; email: gerold.labek@efort.org.
• Francesco Severio Santori, MD, can be reached at Centro Diagnostico, Via Pigafetta 1, 00151, Rome, Italy; +39-06-571071 or +39-34-86434635; email: santori.francesco@gmail.com.
• Dieter C. Wirtz, MD, can be reached at the Clinic of Orthopaedics and Traumatology, University Hospital, Sigmund-Freud-Str. 25, D-53105 Bonn, Germany; +49-228 - 287-14170; email: dieter.wirtz@ukb.uni-bonn.de.
• Luigi Zagra, MD, can be reached at Hip Department, Istituto Ortopedico Galeazzi IRCCS, via R. Galeazzi, 20161 Milan, Italy; +39-0266214825 or +39-0266214735; email: luigi.zagra@rolp.it.
• Disclosures: Haddad, Labek, Wirtz and Zagra have no relevant financial disclosures. Santori is the inventor of the Proxima hip implant (DePuy, a Johnson & Johnson Company).

What are your indications for using short stem total hip arthroplasty?

An option for active patients

Libor Nečas

Historically, the THA procedure was associated with a significant amount of postoperative pain and discomfort and it was not uncommon for patients to have a long recovery. In some cases, patients could suffer a degree of disability after their procedure, meaning they may not resume previous activities.

Patient expectations have changed dramatically. Our patients are more active than ever and more demanding of an outcome that will allow complete resumption of activities. Therefore, one of our main goals should be to minimize the bone loss during surgery while respecting the bone biology and anatomy. From this perspective, short stem implants appear an ideal possibility.

There is a lack of long-term data on short stem implants and we do not know yet how revision rates compare with standard size implant. But, our short- and mid-term results are encouraging. Among the many benefits we see is that, because bone stock is preserved in the initial procedure, a standard size implant can be used should revision of a short-stem THA be necessary.

Libor Nečas, MD, is an orthopaedic surgeon at University Hospital, in Martin, Slovakia and was head of the Slovak Arthroplasty Register in 2003.
Disclosure: Nečas receives contracted research funding from DePuy, a Johnson & Johnson Company.

Many indications

Victor M. Ilizaliturri Jr.

My main indication for a cementless short stem implant is primary hip replacement in Dorr type A and B femurs. This means the metaphyseal region has an almost normal anatomy and is adequate for any type of conventional modern uncemented stem.

A more sophisticated indication is in the presence of femoral deformity: I find short, cementless stems very helpful when proximal third deformities of the femoral shaft are present — for example, patients that had a previous femur fracture and the diaphysis is deformed or the femoral canal is closed or altered due to fracture healing. Short stem implants may also be beneficial when there is hardware from a previous operation in the proximal third of the diaphysis, such as a broken screw.

The design that I use is a flat, tapered, cementless stem with proximal porous coating. The technique that is used for this particular design is very similar to the technique used for a standard size flat, tapered, proximally porous coated stem.

The small size of the short proximally porous coated flat tapered design is also very useful in shorter patients with Dorr type A bone where the internal diameter of the proximal femoral diaphysis is narrow compared with the internal diameter of the metaphyseal region. Because the stem invades less of the diaphysis, it is easier to implant the prosthetic adequately in these kinds of patients.

Victor M. Ilizaliturri Jr., MD, is at the Joint Reconstruction Service, Orthopedics Institute, National Rehabilitation Center in Mexico City, Mexico.
Disclosure: Ilizaliturri receives royalties/research support from Biomet, Smith & Nephew and is a paid consultant to Smith & Nephew. He is a member of the Arthroscopy Association of North America Board of Directors and is Vice President of the International Society for Hip Arthroscopy.