Orthopedics

The Proximal Modular Neck in THA: A Bridge Too Far: Opposes

Hugh U. Cameron, MB, FRCS(C)

  • Orthopedics. 2010;33(9)
  • Posted September 1, 2010

Abstract

Neck/cup impingement is a serious issue, especially with hard/hard bearings. It can produce noise, locking mechanism failure, and an increase in wear debris and dislocation. The double taper neck used by the author has cogs on the neck/stem taper junction for additional rotational stability. One hundred forty-six procedures were performed using the thin mantle cement technique. Mean follow-up was 5 years (range, 3-8 years). A 32-mm neck was used in 73.8% of cases and a 35-mm neck in 26.2%, because most of the patients were elderly women. The neck was anteverted in 1.4%, neutral in 26.4%, and retroverted in the rest (mild in 34.2%, moderate in 14.3%, and maximum in 13.4%). There were no dislocations and no loosenings. Problems were encountered with the neck/stem taper in 3 cases. The stem was therefore taken off the market. The taper was lengthened and the strength doubled.

Since its reintroduction 3 years ago, a further 187 cemented stem procedures have been performed with no failures and no dislocations. Of interest in this series, no necks were anteverted, 23.5% were in neutral, 35.8% were in mild retroversion, 31.1% were in moderate retroversion, and 9.6% were in maximal retroversion. Most necks were placed in retroversion to avoid impingement. This suggests that if a nonmodular neck had been used, some degree of impingement would have occurred in 70% of cases.

The first modular double taper neck was developed by the Cremascoli Company in Italy >20 years ago. The neck stem taper was oval, which gave rotational resistance. A pure double taper with no additional rotational resistance has been tried but did not do well.1 The R120 taper (Encore Medical, Austin, Texas), used by the author, has cogs for additional rotational resistance.2

With most cemented stems available today, the neck is proportional: the bigger the stem, the longer the neck. Unfortunately, osteoporosis is endosteal. This means that as the patients get older, the canal gets bigger. The main indication for cement is type C bone. An article in the literature looked at the Cremascoli modular neck in >2000 cases.3 They showed that without a modular neck, it is not possible to recreate offset, length, and version, especially in women.

Impingement of the neck on the cup was always a problem and could give rise to dislocation and the generation of wear debris. With modern bearing materials, it becomes an even more significant problem. Heavily cross-linked polyethylene has a lower fracture toughness than regular polyethylene, and impingement may result in failure of the locking mechanism. Ceramic liner impingement may produce chipping and noise. Metal liner impingement may result in damage to the neck of the femoral component and generation of metallic wear debris. A modular neck, by allowing changes in version after femoral component insertion, will help reduce impingement.

Another potential advantage of a modular neck is that in a revision, the head and neck can be removed for easy acetabular visualization. A new neck can be inserted, which means that a ceramic head can be used if desired.

The advantages of a modular neck seemed so clear that the author began to use one with a cemented system approximately 7 years ago. This article reviews the results.

The R120 stem is a conventional cobalt chrome stem. It has a collar and a teardrop cement groove and is distally polished. The neck length is 32 or 35 mm with an 8° or 12° twist to allow version change.

The cement technique used by the author is the thin mantle technique, ie, broaching is fairly minimal and a stem is used that is large enough to provide rotational stability…

Abstract

Neck/cup impingement is a serious issue, especially with hard/hard bearings. It can produce noise, locking mechanism failure, and an increase in wear debris and dislocation. The double taper neck used by the author has cogs on the neck/stem taper junction for additional rotational stability. One hundred forty-six procedures were performed using the thin mantle cement technique. Mean follow-up was 5 years (range, 3-8 years). A 32-mm neck was used in 73.8% of cases and a 35-mm neck in 26.2%, because most of the patients were elderly women. The neck was anteverted in 1.4%, neutral in 26.4%, and retroverted in the rest (mild in 34.2%, moderate in 14.3%, and maximum in 13.4%). There were no dislocations and no loosenings. Problems were encountered with the neck/stem taper in 3 cases. The stem was therefore taken off the market. The taper was lengthened and the strength doubled.

Since its reintroduction 3 years ago, a further 187 cemented stem procedures have been performed with no failures and no dislocations. Of interest in this series, no necks were anteverted, 23.5% were in neutral, 35.8% were in mild retroversion, 31.1% were in moderate retroversion, and 9.6% were in maximal retroversion. Most necks were placed in retroversion to avoid impingement. This suggests that if a nonmodular neck had been used, some degree of impingement would have occurred in 70% of cases.

The first modular double taper neck was developed by the Cremascoli Company in Italy >20 years ago. The neck stem taper was oval, which gave rotational resistance. A pure double taper with no additional rotational resistance has been tried but did not do well.1 The R120 taper (Encore Medical, Austin, Texas), used by the author, has cogs for additional rotational resistance.2

With most cemented stems available today, the neck is proportional: the bigger the stem, the longer the neck. Unfortunately, osteoporosis is endosteal. This means that as the patients get older, the canal gets bigger. The main indication for cement is type C bone. An article in the literature looked at the Cremascoli modular neck in >2000 cases.3 They showed that without a modular neck, it is not possible to recreate offset, length, and version, especially in women.

Impingement of the neck on the cup was always a problem and could give rise to dislocation and the generation of wear debris. With modern bearing materials, it becomes an even more significant problem. Heavily cross-linked polyethylene has a lower fracture toughness than regular polyethylene, and impingement may result in failure of the locking mechanism. Ceramic liner impingement may produce chipping and noise. Metal liner impingement may result in damage to the neck of the femoral component and generation of metallic wear debris. A modular neck, by allowing changes in version after femoral component insertion, will help reduce impingement.

Another potential advantage of a modular neck is that in a revision, the head and neck can be removed for easy acetabular visualization. A new neck can be inserted, which means that a ceramic head can be used if desired.

The advantages of a modular neck seemed so clear that the author began to use one with a cemented system approximately 7 years ago. This article reviews the results.

Materials and Methods

The R120 stem is a conventional cobalt chrome stem. It has a collar and a teardrop cement groove and is distally polished. The neck length is 32 or 35 mm with an 8° or 12° twist to allow version change.

The cement technique used by the author is the thin mantle technique, ie, broaching is fairly minimal and a stem is used that is large enough to provide rotational stability without cement. This means that the cement is heavily pressurized. It also means that, like a noncemented stem, there is little version control on the part of the surgeon.

The study group of 146 patients (9 bilateral) comprised 42 men and 104 women. Mean patient age was 73 years (range, 39-87 years). Mean follow-up was 5 years (range, 3-8 years).

Results

There was 1 fatal pulmonary embolism on day 10. There were 2 calcar cracks, which were wired, and 2 greater trochanter fractures, which were treated with screw fixation. There was 1 periprosthetic stem tip fracture at 3 years (stem revised) and 1 stress fracture, which was plated.

A 32-mm neck was used in 73.8% of cases and a 35-mm neck in 26.2% of cases, as the majority of patients were elderly women. An 8° was used in 85.1% and 12° in 13.9%. Again, this reflects relatively normal femoral anatomy.

The version used was anteverted in 1.4%, neutral in 36.4%, and in retroversion in the rest (mild in 34.2%, moderate in 14.3%, and maximum in 13.6%).

There was 1 neck/taper dissociation at 3 years and 2 fractures of the neck/stem taper again at 3 years. In all of these cases, a 35-mm neck had been used, increasing the moment arm. Excluding these cases, a modified Harris Hip Score showed 2.4% were poor and 7.9% were fair, usually due to limp, and the rest were good or excellent. The modification of the Harris Hip Score is to simply ask the patients if they can walk as far as they want.

There were no cases of radiolucent lines or aseptic loosening.

Discussion

The results of the modular neck were promising, with no cases of dislocation. However, there were 3 taper failures, indicating that the design was flawed. The stem was removed from the market and the neck stem taper lengthened. The taper strength was doubled. The implant was reintroduced approximately 3 years ago. Since reintroduction, a further 187 hips have been operated on, with 1 dislocation, no intraoperative complications, and no subsequent problems.

The ease with which leg length can be adjusted post-stem insertion is an advantage. The major advantage, however, is the ability to change version to prevent impingement. Of interest, in the most recent series, the position change was even more marked than on the first series. There were no cases anteverted, and 23.5% of necks were placed in neutral. The neck was placed in mild retroversion in 35.8% of cases, moderate retroversion in 31.1%, and maximal retroversion in 9.6%.

Conclusion

The finding of neck retroversion was surprising initially. Given the version results in the second series, it seems that with more experience, more use is made of this version option.

With the improved length and strength of this taper, the problems of dissociation and fracture appear to have been solved. The author feels that the advantage conferred by the modular neck outweighs the additional complexity, and the author continues to use this stem.

References

  1. Kop AM, Swartz E. Corrosion of a hip stem with a modular taper junction: a review study of 16 cases. J Arthroplasty. 2009; 24(7):1019-1024.
  2. Cameron HU. Cemented femoral fixation: thin mantles, the French paradox. Seminars in Arthroplasty. 2008; (19):144-147.
  3. Traina F, De Clerico M, Biondi F, Pilla F, Tassinari E, Toni A. Sex differences in hip morphology: is stem modularity effective for total hip replacement? J Bone Joint Surg Am. 2009; (91 Suppl 6):121-128.

Author

Mr Cameron is from Orthopaedic and Arthritic Hospital, Toronto, Ontario, Canada.

Mr Cameron has no relevant financial relationships to disclose.

Presented at Current Concepts in Joint Replacement 2009 Winter Meeting; December 9-12, 2009; Orlando, Florida.

“Orthopaedic Crossfire” is a registered trademark of A. Seth Greenwald, DPhil(Oxon).

Correspondence should be addressed to: Hugh U. Cameron, MB, FRCS(C), Orthopaedic and Arthritic Hospital, 318-43 Wellesley St E, Toronto, Ontario, M4Y 1H1 Canada (hugh.cameron@sunnybrook.ca).

doi: 10.3928/01477447-20100722-27

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