This cohort study on the relationship between early repair of hip fracture and in-hospital mortality draws out an (unadjusted) absolute difference in the risk of mortality of 0.57 (95%CI: 0.19 to 0.94) favouring early surgery over delayed. However, patients undergoing delayed surgery were older and had higher severity as measured by the Charlson and the RMI indexes and, therefore, a higher mortality risk. Timeliness on surgery was not found to be related to mortality once confounding factors such as age, sex, chronic comorbidities as well as severity of illness were controlled for in the multivariate analysis. Complementary analysis at hospital aggregated level, multi-level analysis and the instrumental variable approach do not alter these results, which are consistent with most of the studies that have used risk adjustment measures to determine whether patient condition, as opposed to the organization of care, explained the variation in death.
One interesting observation that underpins the idea that other factors, rather than timeliness, are involved in achieving good outcomes after hip fracture surgery is the noticeable decrease in the rate of early surgery over time (a 16% relative reduction in the study timeframe, with an increase of 1 day in the pre-operative length-of-stay), together with a 19% relative decrease in the in-hospital mortality rate. In the same way, a recent study compared the management of hip fracture in Japanese and USA hospitals showing that the length of preoperative stay was very different (1 day in the USA vs. 5 days in Japan) but one-year mortality was similar in both countries . The hypothesis that avoidable mortality in hip fracture is determined by a combination of factors would be consistent with the findings of a recent nationwide cohort study aimed at describing the effect of the quality of care on 30 day mortality after hip-fracture. The study draws out a dose-response effect of being 5.6 times less likely to die if all the 5 agreed quality criteria (as opposed to none) were properly accomplished .
The simultaneous decreasing trends in the practice of early surgery and mortality rates together with the poor discrimination of the regression model on early vs. delayed surgery (Table 3) suggest that unobserved factors related to the care itself are responsible for better results. In fact, one of the better practices throughout Spain in recent years has been the use of tromboembolic prophylaxis, as well as the administration of anti-coagulant drugs or early mobilization. Indirect evidence from a nationwide study in Spain showed a 28% relative reduction in pulmonary tromboembolism and deep venous thrombosis events in hip fracture patients between 2002 and 2005 , underpinning the idea of other interventions rather than the decision on early surgery being behind better results. Organizational changes in the period such as the introduction of integrated clinical pathways [39, 40], various forms of geriatric or medical co-management [41, 42] and a better knowledge of hip fracture syndrome may also have contributed to the decreasing mortality trend.
On the other hand, the surgical alternatives (fixation vs. arthroplasty) which remain in the model as an independent factor in the decision to perform early surgery, together with the already mentioned limited capacity of the model to explain that decision, suggests that the surgical decision process is based on more subtle variables than those used to build the regression model. The impact of this lack of information appears not to affect our results because intervention with one or the other surgical alternative did not affect the risk of death in our sample, but the topic is relevant because different criteria for selecting patients for early or delayed surgery, including surgery modality, could change the relationship between the timing of surgery and mortality. Orthopaedic surgeons and anaesthesiologists use certain, though not always well-defined criteria, to select patients for early or delayed surgery, and it is possible that different subgroups of patients could benefit from one or other surgical strategy.
First of all, our study was observational, retrospective and used administrative databases as the source of information. While criteria for inclusion tried to homogenize the sample and minimize data quality problems, the shortcuts and pitfalls of this data for risk-adjustment are well known . Regarding the main diagnosis, a recent study in the Netherlands has shown that hip fracture may be miscoded by up to the 3% in administrative databases . The paucity of secondary diagnoses recorded (two-third of patients with 3 or less) and the high volume of patients with a Charlson score equal to 0 (our sample only included elderly people, more than 50% over 80 years-old) warns about the possibility of information biases that might affect risk-adjustment. Although Spanish regional departments of health have carried out some internal audits on the Minimum Basic Data Sets in their respective regions, there are very few published studies on data quality in Spain (to our knowledge none of them auditing the hip fracture coding) and its results show problems of accuracy, completeness and information biases [45–47] which are expected to have affected the risk adjustment analysis in some way. Nonetheless, and for the main objective of our study, these problems would suppose an important bias only if there was differential between early or delayed surgery groups and/or with the main study endpoint. As observed in the stratified analysis (Table 4), differences in mortality according to coding strata were not significant, except in the case of patients with more than 6 secondary diagnoses where the death rate was higher in early surgery (a 2.54% absolute difference).
Also related to information biases is the fact that the MBDS do not provide chronological information about secondary diagnoses. While diagnoses for chronic conditions can be confidently treated as comorbidities, acute conditions could be comorbidities or complications. The inability to distinguish among them, together with the possibility of a selective recording of a severe diagnosis depending on the evolution of the patient, reduces the capacity to properly adjust for individual risk. The effect on the estimated differences would be expected to be small, since no differences in mortality were found between any one of the strata, either in the Charlson or RMI Indexes (Table 4). However, the fact that some recorded complications could be either the cause or consequence of delayed surgery does not allow the ruling-out of biased estimates on mortality in the case of this surgical approach. These information biases could tend to overadjust cases with poor outcomes and to attenuate (if they really exist) mortality differences between early and delayed surgery.
The use of inpatient death as the main endpoint (vs. mortality within a fixed time, such as 30 days or one-year from admission, data not available in our study) involves several inconveniences . More aggressive discharge policies can reduce the "mortality" rates if patients are discharged "alive" to die at home. On the contrary, a longer length of stay (LOS) increases the probability of identifying some outcomes such as patient mortality. Because delayed surgery is associated with longer LOS the mortality in this group is probably overemphasized. If poor patient condition was associated with the decision to delay the intervention then these problems could also contribute to overstating mortality in this group. Additionally, the recording of a secondary diagnosis could be associated with LOS and poor outcomes, also affecting the covariates adjustment. The available endpoint does not permit the overcoming of these problems that could bias our results if different discharge policies are used in patients with poor outcomes (associated with one or another of the surgical alternatives evaluated).
One limitation that could potentially affect the estimation of non-differences between early and delayed surgery is the lack of information on the probability of a patient dying in the days immediately following admission. Moreover, the observed cutback in the total length of stay (one day, in spite of the increase in pre-operative stays) over the follow-up period may increase the risk of bias. If we could assume that mortality after discharge followed a random distribution, the estimates would not be affected. We have favoured this possibility by adding "in extremis" patients (i.e. patients who are discharged to die at home) to the case definition, but we are not able to rule out that the hospitals, in our sample, discharge patients depending on the risk of short-term death, thus affecting the non-difference on the timing of surgery.
Finally, the inclusion of only 8 autonomous communities could also be a source of bias if the non-included communities showed different behaviour. Nevertheless, the participating autonomous communities are quite diverse and include practically half of the Spanish population.
Late hip fracture surgery could be due to different reasons. Unavoidable delays in more severe patients unfit for surgery, better control of hemorrhagic risks in patients taking oral anticoagulants or antiplatelet agents, and organizational problems are the most important, being problems caused by the shortage of orthopaedic surgeons or anaesthesiologists very unusual in Spain. With some uncertainty derived from the limitations stated, our results do not show any independent association between early surgery and reduced mortality and does not support the use of early surgery as a quality indicator related with poor outcomes in the Spanish NHS setting. Anyway, and independently of its impact on mortality, while early surgery could shorten the length of stay and its more comfortable for patients, we found a huge variation in the proportion of "early surgery" between hospitals and a surprisingly low average of "early surgery" compared to international standards. These data suggest the need for quality improvement in Spain in terms of good practices in identifying patients that can (or cannot) be operated early. On the other hand, and beyond the delays for organizational reasons, orthopaedic surgeons and/or anaesthesiologists seem to use specific criteria for delaying certain patients. Probably, before developing new studies that analyse the effectiveness of early vs. delayed surgery in all hip fracture patients regardless their clinical status, new research projects should be directed to identify patients with more probability of benefitting from the timing of each surgical approach in the actual context of care for hip fracture patients.