Canal occlusion in cemented primary total hip replacement: autologous compacted bone block compared to a commercially available gelatine plug

Background

It has been universally recognised that a better cementing technique directly improves the longevity of the component and that pressurisation improves the cement/bone interface by interdigitating cement into the endosteal bone. ¹ However, a constant resistance is required distally to sustain cement pressure for interdigitation. ² This is achieved by a cement restrictor or plug. If the plug fails under pressure and moves distally the cement pressure is compromised.^3,4 In-vitro studies have shown that intramedullary pressures during cementation remain between 50 and 75 psi when the restrictor holds its place. So an ideal restrictor should be able to withstand a pressure of at least 50 psi. ⁵ Also, it should not allow any cement egress distally which would again compromise adequate pressurisation. Keeping in mind the possibility of revision surgery it should be biocompatible and should be designed in such a way that it should not pose any problems at the time of revision surgery.

No consensus exists regarding the choice of restrictors in clinical practice. There are limited in-vivo studies comparing different plug designs and materials.^6–10

The possibility of a future revision surgery mandates the use of a revision-friendly plug which does not pose difficulties during the revision surgery. Bio-absorbable/bio-degradable plugs and polymethylmethaacrylate (PMMA) plugs, both have this advantage. In our clinic 2 types of absorbable plugs are used – autologous compacted bone block and the gelatine C-plug (DePuy Synthes, Leeds, UK). Bio-absorbable gelatine plugs had a surge in their use following the concerns raised against the use of polyethylene restrictors recently.^10–12 Autologous bone blocks, on the other hand were popularised by Wroblewski et al. ¹³ but were not commonly used following the introduction of polyethylene restrictors. However, they are perceived to have certain advantages. Firstly, they are autologous being derived from the patient’s own bone which makes them costless. Also, they are absorbed in most cases. In case of bony incorporation into the canal, they can easily be drilled through during revision surgery and do not pose any risk of perforation unlike the polyethylene restrictors. But the evidence in the literature has raised questions on their performance. Beim et al. ¹⁴ and Smith et al. ¹⁵ in their in-vitro studies have reported poor outcomes with bone blocks but both studies suffer from inadequate sample size and no statistical analysis. A prospective randomised controlled trial by Thomsen et al. ¹⁶ reported bone blocks to be inferior but again with inadequate sample size of 23–29 patients in each group. Hence, any good quality evidence comparing the 2 types of plugs and their individual performance was lacking which incited this comparative study.

We conducted this study to evaluate the stability of these 2 plugs and the quality of cementation. The study was driven by the clinical observation of significantly longer cement tails following cessation of use of the Ceraplug (Ceraver-Osteal, Roissy, France) and introduction of C-plug (DePuy Synthes, Leeds, UK) in our institute.

Materials and methods

A retrospective radiographic comparative review was designed and executed after approval from the local Research Department. A sample size calculation was based on previously reported data on cement tail length and variation.^8–10 The standard deviation of migration of bone block is quoted to be 13.8 mms and for migration of a biodegradable plug to be 20 mm.^8,9 Using the above values, a sample size of 74 patients in each arm was calculated assuming a power of 90% and a significance level of 5%.

Surgical technique

All patients were operated on using the same surgical approach and operative technique other than the choice of cement restrictor. Femoral canal preparation involved use of serial broaches and reamers, pulse lavage of the canal and thorough cleaning and drying. The same viscosity cement was used for all participants and third generation cementing was used. The cement was delivered with a standard small-bore cement gun by retrograde filling and standard silicone proximal pressurisers were used in all the cases.

The operating surgeons have all been trained in the same technique and aimed to achieve femoral cementation in such a way that there is no more than 1 cm of cement distal to the tip of the stem.

The bone block was retrieved using a trephine either from the neck of femur or from the resected head (Figure 1). The bone harvested in the trephine is then compacted with tamps within the trephine to increase the density and the strength of the plug. The canal size is measured using the serial tampers (largest to smallest; 14, 10 and 8 mm) and then the compacted bone block of diameter 15 mm (inner diameter of trephine) is inserted to the determined level depending upon the length of the stem aiming for minimal (<10 mm) cement beyond the tip of the cement (Figure 1). The depth of insertion of the plug was measured intraoperatively depending on the length of the stem to be used.

OPEN IN VIEWER

Figure 1.

Standard Wroblewski instruments to harvest a bone block.

The C-plug is a bio-absorbable plug made mainly of gelatine which is available in diameters of 10–18 mm in 2-mm increments (Figure 2). The canal is measured using sounds of increasing diameter, followed by insertion of the plug to a pre-measured depth, again depending on the length of the stem used. The size of the plug used is 1 size larger than the measured diameter of the femoral canal at the desired level and is inserted to the determined level depending upon the length of the stem aiming for minimal (<10 mm) cement beyond the tip of the cement.

OPEN IN VIEWER

Figure 2.

C-plug – bioabsorbable plug available in diameters of 10–18 mm in 2-mm increments.

Results

Discussion

Cement restriction is a critical step in cemented arthroplasty which contributes to the longevity of components. Restrictors, or plugs, made of different materials and of different shapes have been tested in-vitro and in-vivo in cemented arthroplasty.^{6,8–11,15,16,18} Currently available restrictors include autologous bone blocks, polyethylene restrictors, polymethyl methacrylate (PMMA) plugs and gelatine biodegradable plugs. Plugs of a single material are available in varying designs.

Initially autologous bone blocks from the femoral canal/head were used as cement restrictors. Wroblewski et al. ¹³ devised an effective instrumentation set and described the retrieval technique for harvesting the bone blocks. Subsequently, artificial cement plugs/restrictors made of synthetic material such as polyethylene and PMMA were introduced. Studies carried out in the late 80s and early 90s concluded that the stability and resistance offered by the artificial polyethylene restrictors was superior to that offered by autologous bone blocks.^14,16 The increase in revision surgeries led to the introduction of biodegradable plugs made of gelatine. Studies comparing these biodegradable plugs with the conventional polyethylene plugs have shown that the biodegradable plugs are less competent to intra-medullary pressures and are associated with higher rates of migration.^11,18 PMMA plugs of certain designs have been reported to perform better than the polyethylene and the gelatine plugs in some studies,^7,14,15 however, their performance was dependent on the flexibility and elasticity of the specific plug. Some PMMA plugs like the Palacos plug (Merck Biomaterial, Germany) are rigid and non-flexible and have been shown not to sufficiently occlude the canal. ⁷ Specifically, when the canals are distally expanding or the plug is to be placed distal to the isthmus, these rigid PMMA plugs were associated with significant migration and cement leakage distally, whereas the flexible plugs like the PMMA Exeter plug (Stryker Howmedica, Mahwah, NJ, USA) and the gelatine plugs could conform to the dimensions of the canal. PMMA plugs are directly in contact with the cement and can be drilled through at the time of revision and do not add significant difficulty at the time of revision.

There are no national guidelines available to help surgeons make the choice of restrictor. Furthermore, the available literature is not conclusive. The common conclusion which can be drawn from the available trials and reports suggest that the absorbable gelatine plugs are inferior in strength and performance as compared to the polyethylene plugs. The ultra-high molecular weight polyethylene (UHMWPE) ‘finned’ restrictor which was commonly known as the Hardinge restrictor was once the most commonly used restrictor. However, issues were raised against its suitability. The work done by Black et al. ¹² found that significant number of polyethylene particles are produced off the fins of the restrictor and they are of appropriate size for phagocytosis. These particles may be implicated in periprosthetic osteolysis and hence non-polyethylene restrictors were introduced. However, Wembridge and Hamer ¹⁰ and Visser et al. ¹¹ reported significantly less migration with the use of Hardinge restrictor when compared to bio-absorbable gelatine restrictors but also reported significantly increased cement leakage associated with Hardinge restrictor. This leakage was associated with fin breakage which allowed cement egress into the canal beyond the restrictor thereby compromising cement pressure. Moreover, these polyethylene restrictors mandate removal during a revision scenario. These issues led to a decrease in the use of Hardinge restrictors and bioabsorbable gelatine restrictors became more popular.

Bone blocks, as an alternative, have advantages of being autologous, revision-friendly and cost-free. However, a few studies in the literature have reported poor results with the use of bone blocks.^14–16 To address this question further we conducted this comparative review between the bone blocks and absorbable gelatine C-plugs.

In this study the bone blocks were associated with significantly shorter cement tail lengths and demonstrated increased resistance to cement pressures. This was found in a statistically sufficient sample in contrast to the previously reported studies. This could perhaps be attributed to the increased resistance to cement pressure which is achieved because of high friction between the bone block and the bone in the medullary canal. The modification of the technique to include compaction of the bone block into the harvesting trephine with tamps before insertion into the canal increases the density of the plug, improving stability when impacted into the canal.

A negative correlation was found between the length of the cement tail and the quality of cementation which emphasises that the stability of the restrictor is directly related to the quality of cementation. This has also been reported by Vanderstappen et al. ⁹ Excellent quality of cementation (Barracks Grade A) was reported in 81% of patients in the bone block group. This was better than that described in series using polyethylene or gelatine restrictors.^9,15,18

Though retrospective, this study is the only available series which compares the 2 types of plugs with an adequate sample size. This allows us to apply the obtained results in the general population. The analysis was performed by 2 independent observers, which further adds to the strength and applicability of this study. Though multiple surgeons were involved in both groups, it reinstates that the technique of bone block harvesting is easily reproducible.

There are some limitations associated with this study. Although the desired depth of insertion of the restrictor was measured intra-operatively based on the length of the stem and a minimal cement tail length of less than a centimetre, this was not confirmed after the insertion of the plug. It was, therefore, assumed in the study that the longer cement tail was because of the migration of the restrictor from its original position as the pre-cementation position of the restrictor was not recorded intraoperatively. Also, it is difficult to account for an inevitable observer bias which could have been introduced at the time of analysis as the cement tail lengths and Barrack’s grade were determined using the same radiograph. A prospective randomised controlled trial with adequate sample size with adequate calculation of the migration of plugs would be needed to address these limitations and would further substantiate the evidence on this. Although the type of cement and the cementing technique used was uniform throughout all the cases, the timing of introduction of cement could have been variable. This could not be analysed as it was not recorded in the operation notes and was considered to be similar in all cases after discussions with the involved surgeons regarding the usual time interval they wait for before introduction of the cement. The surgical time required in the bone block group was higher (not statistically significant), however, it can be because of a number of other variables and the involvement of different surgeons.

Despite the above-mentioned limitations this study contributes significantly to the existing evidence on the use of cement restrictors in cemented primary hip arthroplasty. Although the bone block has been found to be inferior in stability by some previous studies,^14,16 this may be attributed to the inconsistent technique and insufficient sample sizes. It is further emphasised to surgeons using the bone block technique that the compaction of the bone into the trephine before placing it enhances its quality, density and strength and may be responsible for improved performance of bone blocks in this study when compared to previous studies in literature.

We have shown that better quality cementation and shorter cement tails can be achieved with the cheapest of all options for canal occlusion, an autologous compacted bone block and hence recommend this technique.