Bioabsorbable versus metal screw for graft fixation in the surgical treatment of anterior cruciate ligament injury

Background

The anterior cruciate ligament (ACL) is a knee ligament that functions to stabilise the knee. ACL injuries are more common in athletes, such as football, basketball and handball players. Many people with ACL injuries are treated with surgery to reconstruct this ligament. In ACL reconstruction, a replacement ligament (graft) is attached to tunnels drilled into the end of the femur (thigh bone) and tibia (shin bone). Often screws are used to attach the graft to the bone. Traditionally, metal screws have been used. Although these are generally successful, metallic screws can be hard to remove if further surgery is required. They also interfere with looking at the knee using magnetic resonance imaging (MRI). With the aim of avoiding these disadvantages, and in response to patient requests, screws made ​​from materials that dissolve over time (bioabsorbable screws) were introduced. However, such screws have been reported to have increased risks of inflammation, infection, and failed surgery.

Results of the search

We searched medical databases up to January 2016 for randomised studies comparing bioabsorbable with metal screws for graft fixation. We included 12 studies involving 944 participants undergoing surgery (ACL reconstruction).

Key results

We found evidence that self-reported measures of knee function were similar at one, two and five of more years in those treated whose grafts were fixed with bioabsorbable screws to those whose grafts were fixed with metal screws. Similarly, no differences were seen between the two types of screws in levels of activity at one and two years. However, there was evidence that bioabsorbable screws may be associated with more treatment failures. These include screw breakage during surgery, and greater numbers of graft rupture.

Quality of the evidence

All 12 studies had weaknesses that could affect the reliability of their results. We considered the evidence was very low quality meaning that we are unsure of these results.

Conclusions

The limited evidence does not show that knee function and activity levels are any better after bioabsorbable screws compared with metal screws. However, such screws may break during surgery and may result in a greater risk of later treatment failure. Further research does not appear to be a priority but if undertaken should also examine costs.

Authors' conclusions: 

There is very low-quality evidence of no difference in self-reported knee function and levels of activity between bioabsorbable and metallic interference screws for graft fixation in ACL reconstruction. There is very low-quality evidence that bioabsorbable screws may be associated with more overall treatment failures, including implant breakage during surgery. Further research does not appear to be a priority, but if undertaken, should also examine costs.

Read the full abstract...
Background: 

Anterior cruciate ligament (ACL) tears are frequently treated with surgical reconstruction with grafts, frequently patella tendon or hamstrings. Interference screws are often used to secure the graft in bone tunnels in the femur and tibia. This review examines whether bioabsorbable interference screws give better results than metal interference screws when used for graft fixation in ACL reconstruction.

Objectives: 

To assess the effects (benefits and harms) of bioabsorbable versus metallic interference screws for graft fixation in ACL reconstruction.

Search strategy: 

We searched the Cochrane Bone, Joint and Muscle Trauma Group Specialised Register, CENTRAL (the Cochrane Library), MEDLINE, Embase, LILACS, trial registers and reference lists of articles. Date of search: January 2016.

Selection criteria: 

We included randomised controlled trials and quasi-randomised trials comparing bioabsorbable with metallic interferences screws in ACL reconstruction. The main outcomes sought were subjective-rated knee function, failure of treatment, and activity level.

Data collection and analysis: 

At least two review authors selected eligible trials, independently assessed risk of bias, and cross-checked data. Data were pooled whenever relevant and possible. Requests for further information were sent to the original study authors.

Main results: 

We included 12 trials (11 randomised and one quasi-randomised) involving a total of 944 participants, and reporting follow-up results for 774. Participants in the 12 trials underwent ACL reconstruction with either hamstring tendon grafts (five trials) or patellar tendon grafts (seven trials). Trials participants were randomly allocated to bioabsorbable or metallic interference screws for graft fixation in both femur and tibia (seven trials); femur only (three trials); tibia only (one trial); location was not reported in the remaining trial. A variety of materials was used for the bioabsorbable screws, Poly-L-lactic acid (PLLA) being the most common. The metallic screws, where reported, were titanium.

All trials were at high risk of bias, which invariably included performance bias. Seven trials were at high risk of attrition bias and eight at high risk of reporting bias. The quasi-randomised trial was assessed as being at high risk for selection bias. Based on these study limitations and insufficiency of the available data, we judged the quality of evidence for all outcomes was very low.

The majority of the available data for patient-reported knee function was presented as Lysholm scores (0 to 100; higher scores = better function). There was very low quality but consistent evidence of no clinically important differences between the two groups in Lysholm scores at 12 months follow-up (mean difference (MD) -0.08, 95% confidence interval (CI) -1.48 to 1.32; three trials, 168 participants); 24 months (MD 0.35, 95% CI -1.27 to 1.98; three trials, 113 participants) or five or more years follow-up (MD 1.23, 95% CI -2.00 to 4.47; two trials, 71 participants). This lack of between-group differences was also reported for Lysholm scores in several trials that did not provide sufficient data for pooling as well as for other self-reported knee function scores reported in several trials.

Treatment failure was represented by the summed data for implant breakage during surgery and major postoperative complications (implant failure, graft rupture, symptomatic foreign body reactions, effusion and treated arthrofibrosis and related conditions) that were usually described in the trial reports as requiring further substantive treatment. There is very low-quality evidence of greater treatment failure in the bioabsorbable screw group (60/451 versus 29/434; risk ratio (RR) 1.94 favouring metallic screw fixation, 95% CI 1.29 to 2.93; 885 participants, 11 studies). In a population with an assumed risk (based on the median control group risk) of 56 participants per 1000 having treatment failure after metallic screw fixation, this equates to 53 more (95% CI 17 to 108 more) per 1000 participants having treatment failure after bioabsorbable screw fixation. All 16 intraoperative complications in the bioabsorbable screw group were implant breakages upon screw insertion. Treatment failure defined as postoperative complications only still favoured the metallic screw group but the 95% CI also included the potential for a greater risk of treatment failure after metallic screw fixation: 44/451 versus 29/434; RR 1.44, 95% CI 0.93 to 2.23. Based on the assumed risk of 56 participants per 1000 having postoperative treatment failure after metallic screw fixation, this equates to 25 more (95% CI 4 fewer and 69 more) per 1000 participants having this outcome after bioabsorbable screw fixation.

There was very low-quality evidence of very similar activity levels in the two groups at 12 and 24 months follow-up measured via the Tegner score (0 to 10; higher scores = greater activity): 12 months (MD 0.08, 95% CI -0.39 to 0.55; 122 participants, two studies); 24 months (MD 0.01, 95% CI -0.54 to 0.57; 72 participants, two studies).

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