Technical Note for Transfemoral Implantation of Tapered Revision Stems. The Advantage to Stay Short.

BACKGROUND: The aim was to test the hypothesis that during transfemoral implantation of a conical revision stem, the fixation of the stem at the distal tip leads to a low rate of periprosthetic fractures.
MATERIAL AND METHODS: Two hundred eighty-two stem revisions by a transfemoral approach in cases of Paprosky Type II and IIIA-defects (with a sufficient isthmus) were carried out and analyzed during and radiographically after the surgery for unintentional periprosthetic fractures below the osteotomy.
RESULTS: In all cases, fixation was always achieved at the tip of the distal component in the isthmus of the femur. No periprosthetic fractures were observed.
CONCLUSIONS: When the isthmus of the femur is intact, a transfemoral implantation of a tapered revision stem at the distal end reduces the risk of periprosthetic fractures by preventing bypassing the isthmus with the stem. Knowing the difference between the nominal diameter and the diameter at the distal start of the conical zone can help to create this fixation technique resulting in short revision stems.

Introduction

Distally fixed, cementless tapered revision stems represent a successful concept for the revision of hip prostheses [[1], [2], [3], [4], [5], [6], [7], [8]]. This type of revision stem is available in a monobloc form or in a modular version, the former with a straight stem and the latter with a straight or curved stem. A common feature for most versions is that a conical reamer (for a straight stem) or a conical rasp (for a curved stem) is used to create a conical anchorage bed in the femur. The conical stem is inserted into this bed so that a firm cone-in-cone fixation is achieved [5,9].

The implantation can either be done endofemoral or transfemoral (with an extended trochanteric osteotomy). In transfemoral revisions with an intact isthmus of the femur, cone-in-cone fixation in the isthmus takes place over a distance defined by the preparation (with the conical reamer of rasp) [9,10]. In 2 degree tapered stems, this fixation can take place at the distal end of the stem or the distal component (in modular stems) (Fig. 1a-c) [9,10]. The use of longer stems or distal components does not lead to a longer fixation zone, but to an equally long fixation zone further proximally on the stem (Fig. 2) [10]. This in turn leads to the stem passing through the isthmus, which increases the rate of unintentional fractures, especially when straight revision stems are used [10]. Many authors have given fracture rates with conical revision stems of over 10% [[11], [12], [13], [14], [15], [16], [17], [18], [19], [20]], in some cases even over 20% [[21], [22], [23], [24]]. These fracture rates should be able to be reduced by fixing the stem in the isthmus without bypassing the isthmus with the stem. The aim of this work was therefore to test the hypothesis that during transfemoral implantation of a conical revision stem, the fixation of the stem at the distal tip in the isthmus resulting in shorter stems leads to a low rate of unintentional periprosthetic fractures.

Figure 1

(a) A 73-year-old man with infected well-osteointegrated cementless hip arthroplasty on the right side. (b) Postoperative radiograph after transfemoral reimplantation of a modular revision stem Revitan curved and a press-fit cup (Allofit S; ZimmerBiomet, Winterthur, Switzerland) 6 weeks after transfemoral removal of the infected implants and temporary spacer placement. (c) Radiograph done 2.5 years postoperatively showing a complete osteointegration of the implants and healing of the approach and osteotomy.

Figure 2

Comparison of a shorter and longer distal component concerning the position of the fixation zones in the isthmus of the femur at the stem.

Material and Methods

Two hundred eighty-two stem revisions by a transfemoral approach using a previously published modified Wagner technique in cases of Paprosky Type II and IIIA-defects (with a sufficient isthmus) were carried out between July 2004 and June 2018 [25,26]. These were 151 women and 131 men with an average age of 67.1 ± 10.1 (44 - 95) years and a BMI of 28.7 ± 5.1 (19.3-40.9). These were 194 aseptic and 88 septic revisions; 110 times the first revision, 122 times the second, 40 times the third, and 10 times the fourth revision. The indication for a transfemoral approach was determined during preoperative planning. According to the previous published indications of Paprosky et al. [27,28] and Fink et al. [25,26,29,30], the transfemoral approach was performed in the following conditions: always in cases of broken endoprosthesis stems (5 cases) or only partially loosened cementless stems with a coarsely porous structure (33 cases) to remove the stem. In cases of cemented stems, where the cement could not be completely removed using the endofemoral approach, the transfemoral approach was also adopted for the surgical operation (56 cases). A transfemoral approach was also chosen during preoperative planning when the femoral axis was so deformed by loosening of the prosthesis that it had to be corrected by osteotomy (63 cases) [[25], [26], [27], [28]]. Furthermore, periprosthetic fractures, where the implant was loose, were revised via a transfemoral approach (37 cases) [29,30]. In septic two-stage revisions, the transfemoral route was used to remove the infected but well-anchored cementless stems or well-cemented stems with septic osteolysis [31]. In these 88 cases, the transfemoral approach was reopened in the second stage to replace the spacer with the new prosthetic implant.

The transfemoral approach was carried out using a previous published modified Wagner technique [25,26,30]. After a posterolateral incision, the posterolateral edge of the femur ventral to the linea aspera was exposed in the septum intermusculare laterale after ligation of the perforating vessels. The lateral circumference of the femur was exposed in the area where the end of the osteotomy flap was going to be positioned and 2 3.2-mm holes drilled under cooling (above the linea aspera and 180 degrees ventromedial from the first hole) (Fig. 3a). The ventromedial trochanter region was osteotomized using a chisel at the vasto-gluteal border, and then the dorsolateral osteotomy, the connecting osteotomy between the 2 drill holes, and the distal ventromedial osteotomy of about 3 cm were performed with a water-cooled oscillating saw (Fig. 3a). The ventromedial osteotomy was completed with a chisel that was introduced into the already prepared distal, ventral osteotomy and then driven blind under the vastus lateralis muscle to the proximal end of the osteotomy. The flap with the vastus lateralis muscle attached was opened in a ventromedial direction (Fig. 3b). Before preparing the fixation bed in the isthmus of the femur, a prophylactic double cerclage wire distal to the osteotomy was placed (Fig. 3b). After implantation of the revision stem, the closure of the bone flap was achieved most of the time with the aid of 2 double cerclages using 1.5-mm cerclage wire (Fig. 1b and c).

Figure 3

(a) Diagram showing the localization of the flap of the modified transfemoral approach. (b) Diagram showing the opened flap of the transfemoral approach.

The modular cementless revision system Revitan Curved (ZimmerBiomet GmbH, Winterthur, Switzerland) was used in all cases. It is based on the principles of the Wagner SL-stem (ZimmerBiomet GmbH, Winterthur, Switzerland) and consists of a distal curved component with a thickness ranging from 14 mm to 28 mm (in 2-mm steps) and lengths of 140 mm, 200 mm, and 260 mm which can then be combined with the proximal component with a length ranging from 55 mm to 105 mm (in 10-mm steps). The curved distal component has an octagonal cross-section and is tapered at an angle of 2 degrees. The rotational stability of the implant is achieved by the spline peaks of the octagonal form that run along the whole length.

Because of the known difference between the diameter at the distal start of the conical area and the nominated stem thickness of 3.8 mm after reaming the cortical canal with a flexible reamer until solid cortical contact was reached, a distal component of 140 mm length (in 276 cases) or 200 mm length (in 6 cases) and a nominated thickness of 4 mm larger than the last reamer was inserted. This was done after stepwise preparation of the conical femoral fixation zone with the conical rasps (last rasp of 4 mm lager than the last reamer). By this, a fixation zone of 3 to 5 cm at the tip of the distal component in the isthmus of the femur was reached (Fig. 1a-c).

Postoperatively, the leg was subjected to partial weight-bearing by loading with 10 to 20 kg for a period of 6 weeks. Thereafter, the weight-bearing was gradually increased to full weight-bearing 3 months postoperatively similar to how other authors did for other cementless revision stems [1,4,6,14]. The hip joint was not allowed to be flexed for more than 70° for 6 weeks after the operation to avoid movement of the bone flap [10,26].

The incidence of periprothetic fractures was determined intraoperatively and postoperatively by radiological examination. All patients were followed up for at least 2 years for radiological analysis of loosening or subsidence. Five patients (with 5 revision stems) died from causes unrelated to the revision operation before the 2-year minimum follow-up period was reached, and 5 patients were lost to follow-up, so that a total of 272 stems from 270 patients were followed. Subsidence of the stem was assessed by comparing all the postoperative radiographs using the technique described by Challaghan et al. [32] and McInnis et al. [22]. Hereby, vertical subsidence of the femoral component was measured as the change in the distance from the inferior margin of the component neck to the most proximal point on the lesser trochanter and from the proximal lateral end of the component body to the tip of the greater trochanter. Any subsidence greater than 5 mm was classified as significant in accordance with the study by Pattyn et al. [3] and van Houwelingen et al. [6]. The osteotomy site was considered to be radiologically healed if callus was seen bridging the site in both the anteroposterior and lateral planes in agreement with the study by Chen et al. [33] and Miner et al. [34]. The mean follow-up period was 7.44 ± 2.09 years (2 - 15 years). All subjects gave informed consent to participate in the study, and the protocol was approved by the research ethics board of the respective institution.

Results

In all 282 cases, fixation was always achieved at the tip of the distal component in the isthmus of the femur (Fig. 1a-c). The mean circular press-fit fixation zone at the tip of stems implanted was 4.13 ± 0.86 cm (3.0-5.3 cm). The mean length of the flap created during the transfemoral approach was 17.7 ± 3.3 cm (12.2-24.0 cm). The mean length of the implanted stems was 214.9 ± 16.4 mm (195-265 mm), and the mean nominal diameter (name of the thickness of the stem) was 18.6 ± 2.5 mm (14-24 mm).

No unintentional periprosthetic fractures below the transfemoral approach which would weaken the fixation zone of the new revision stem were observed. During implantation of the original distal component, a fissure at the beginning of the isthmus was observed 8 times (2.8%). They were treated with additional double cerclage wires and healed without subsidence of the stem. Nonprogressive subsidence was seen in 6 of the 272 cases with follow-up examination (2.2%). There was no case of loosening. Nonunion of the bony flap was seen in 2 cases (0.7%) of a two-stage septic revision. Other complications included 7 dislocations (2.6%) that could be treated conservatively in 5 cases and needed revision surgery in 2 cases as well as 3 cases of thrombosis (1.1%).

Discussion

This study was designed to investigate whether the consistent fixation of tapered revision stem at the distal tip in an intact isthmus of the femur leads to low rate of periprosthetic fractures. We could show that in these defect types of Paprosky II and IIIA, short stems with the short distal component of 140 mm length can be used on a regular basis to achieve reproducibly good results with preventing unintentional periprosthetic fractures and a very low rate of fissures. Moreover, as in previous studies, we could show that with this technique also reproducible good results concerning aseptic loosening and subsidence of the stem could be achieved [5,10]. Russell et al. [35] concluded in a biomechanical study that 1.5- to 2.5-cm cone-in-cone-fixation at the tip of a tapered revision stem (Wagner SL with 2 degree taper) seems to result in sufficient stability. We showed for the clinical use of the Revitan curved stem (with 2-degree taper) that the minimum fixation-zone seems to be 3 cm [5,36].

The constant fixation at the tip of the stem could be achieved in our study by knowing the difference between the nominal diameter and the diameter at the distal beginning of the conical zone. For the stem we used, this difference was 4 mm (exact 3.8 mm). This principle should also work when using other conical revision stems if the difference between the nominal diameter and the distal start of the conical segment is known. According to the companies, we have therefore summarized this difference for the shorter distal components of the common tapered revision stems in Table 1. In order to implement this procedure and operative technique even with tapered straight stems, the isthmus would first be cylindrically reamed with flexible medullary reamers to determine the stem thickness. The conical fixation bed for the cone-in-cone fixation of the revision stem would then be created with the conical reamers for the straight stems. However, it must be taken into account that the fixation starting at the distal tip of the stem is only possible with 2-degree tapered stems. In the case of stems with a higher degree of taper, the fixation zone cannot begin directly at the distal tip. The higher the degree of conicity is, the more it migrates proximally (Fig. 4). Thus, even with higher degrees of conicity, the fixation cannot begin directly at the distal tip and thus for the nominal diameter of the selected distal component, the whole difference to the diameter at the distal end of the conical segment cannot be added completety. For example, the difference between the nominal diameter and the diameter at the distal tip of the ZMR stem (with a taper of 3.5 degrees) is 6.1 mm or 6.0 mm. Here the fixation cannot begin directly at the tip of the stem. Therefore, the selected stem will be nominally 5 or 4 mm thicker than the last medullary reamer which had a solid cortical contact in the isthmus. Using this technique results in extension of the stem from the distal end of the osteotomy of 3 to 5 cm with a 2 degree tapered stem (Fig. 1b and c) and in an increasing extent with increasing conicity of the stem (Fig. 4) [9,10,36]. The fixation zone of the stem can be controlled intraoperatively with an imaging intensifier.

Table 1

Features of different monoblock and modular tapered revision stems.

Stem	Company	Modularity	Straight/curved	Degree of conicity	Difference of nominal diameter and the diameter at the distal start of the conical area
Arcos	Zimmer-Biomet (Warsaw, IN)	Modular	Straight	3	150 DC: 4.17 mm for ND 12 to 3.70 mm decreasing for ND 30190 DC: 6.32 mm fori ND 12 to 5.85 mm decreasing for ND 30
MRP	Peter Brehm (Weisendorf, Germany)	Modular	Straight 80, 140, 200 curved	2	4 mm for 80, 140, and 200 DC
MP	Waldemar Link (Norderstedt, Germany)	Modular	3° Kinked	2	2.5 mm
Mutars R	Implantcast (Buxtehude, Germany)	Modular	Curved	1,5	4 mm for 150 and 200 DC
Prevision	Aesculap (Tuttlingen, Germany)	Modular	StraightCurved	20,6	1 mm
Profemur R	MicroPort Orthopedics (Shanghai, China)	Modular	135 DC mm straight175 and 215 mm DC curved	2	0 mm
Reclaim	DePuy Synthes (Warsaw, IN)	Modular	140 mm straight190 mm straight and 3° kinked240 and 290 mm 3° kinked	2,5	4 mm for 140 DC6 mm for 190 DC
Redapt	Smith & Nephew (London, GB)	Mono-block + with Sleeve	Straight	3	3 mm for 190 stem5 mm for 240 stem
Restor-ation	Stryker (Kalamazoo, MG)	Modular	Straight	3 center2 rips	4 mm
Revitan	Zimmer-Biomet (Winterthur, Switzerland)	Modular	Straight and curved series	2	3.8 mm
Wagner SL	Zimmer-Biomet (Winterthur, Switzerland)	Mono-block	Straight	2	3.8 mm
ZMR	Zimmer-Biomet (Warsaw, IN)	Modular	Straight	3,5	6.1 mm for ND 17-196.0 mm for ND ≥ 20

DC, distal component in modular stems; ND, nominal diameter.

Figure 4

Comparison of the fixation zone at the distal component between a 2-degree tapered stem (left) and a distal component with 3.5 degree taper (right).

The technical rule (of +4 mm in the stem we used) can only be used when implanting the stem via a transfemoral procedure. Owing to the different shapes of the femoral bone and the implanted stems (especially in straight stems), the difference between the last flexible reamer and the original stem is individual for endofemoral implantation (2 or 3 mm in the Revitan Curved).

In conclusion, it can be said when the isthmus of the femur is intact, a transfemoral implanting of a tapered revision stem at the distal end reduces the risk of periprosthetic fractures by preventing bypassing the isthmus with the stem. Knowing the difference between the nominal diameter and the diameter at the distal start of the conical zone can help create this fixation technique resulting in short revision stems.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this article. The author is a consultant for ZimmerBiomet.