This study, the largest to date on hip fractures in the super-elderly population, used a comprehensive database of hip fractures in the elderly in Japan to analyze whether hip fractures in the super-elderly were associated with the incidence of pneumonia, pulmonary embolism, myocardial infarction, acute renal dysfunction, urinary tract infection, subsequent cognitive dysfunction, and in-hospital mortality. The results of this study showed that compared to hip fractures in the elderly aged 65 to 89, hip fractures in the super-elderly aged 90 and over were associated with an increased risk of pneumonia, urinary tract infection, acute renal dysfunction, subsequent cognitive dysfunction, and mortality during hospitalization, after adjusting for confounding factors. The odds ratios were 1.636 (95% CI 1.565–1.709), 1.683 (95% CI 1.399–2.026), 1.378 (95% CI 1.321–1.438), 1.378 (1.321–1.438), and 2.190 (95% CI 2.062–2.325), respectively. In addition, although 30-day survival was good at 99.1% for the elderly and 97.9% for the super-elderly, an association was found between being over 90 years of age and increased mortality during hospitalization. As hip fractures in the super-elderly are associated with a risk of serious subsequent complications and death, it was confirmed that prevention of falls and osteoporotic fractures, as well as early surgery after a fracture, are important.

There have been reports on the risk of mortality after a hip fracture, including advanced age, male sex, elective surgery after 48 h of injury, dementia, and chronic renal function [21,22,23,24,25,26]. This study also found that these variables were independently associated with the risk of mortality. Pneumonia-induced respiratory failure and myocardial infarction are reported to be the most common causes of death [15, 27]. In this study, the 30-day survival rate for the super-elderly group was 97.1%, which was a good result compared with previous overseas studies. Japan's medical system, which is making progress in dealing with an aging society, is thought to be becoming more adept at dealing with hip fractures in the super-elderly, such as perioperative management. However, this study did not examine the course of events after discharge from the hospital, and it did not examine the long-term prognosis, such as the more important one-year mortality rate, so it is necessary to clarify this in future cohort studies that also include data after discharge from the hospital.

The analysis of this study revealed details of hip fractures in Japanese super-elderly population. In the super-elderly group, the proportion of femoral neck fractures decreased (36.6%), while the proportion of femoral trochanteric fractures increased (61.2%) compared to the elderly group. The proportion of patients who underwent surgery within two days of admission was 51.1%, which is higher than the rate for the elderly group (47.1%). The implementation of antithrombotic treatments to prevent thrombosis and pulmonary embolism was 49.2% for both the super-elderly group and the elderly group, suggesting that the importance of preventing thrombosis is recognized within the Japanese healthcare system, even for the super-elderly population. However, since pneumonia, pulmonary embolism, and mortality rates during hospitalization are high in the super-elderly group aged 90 and over, it is hoped that in the future, hip fracture patients in this super-elderly group will receive even faster surgery and more thorough antithrombotic treatments to prevent embolism. There have been reports that the intervention of a multidisciplinary medical team before surgery was useful in reducing complications and shortening the length of hospital stay for elderly patients with hip fractures [28]. On the other hand, the use of bisphosphonates for the treatment of osteoporosis is low even among super-elderly patients, and the use of osteoanabolic agents and anti-RANKL antibodies, which are more effective in the treatment of osteoporosis than bisphosphonates [29, 30], is also low, suggesting that the treatment of osteoporosis is not yet sufficiently widespread. Because hip fractures in the super-elderly are associated with a high risk of serious complications, it is important to prevent them by thoroughly treating osteoporosis and promoting fracture liaison services [31].

There are several limitations to this large study, which will be discussed below. First, the study population included patients with hip fractures who were treated exclusively in acute care hospitals and reported in the DPC data system. This excludes patients admitted to non-DPC-reported beds, which represent 30% of all general hospital beds, or patients never treated in an acute hospital [16]. Secondly, the limitations of this study include the inability to validate the names of DPC diseases and the inability to assess the severity of symptoms in actual patients. Third, although conservative treatment may be chosen instead of surgery for super-elderly patients with hip fractures, this study only includes patients with surgical treatment, so the outcomes of conservative treatment in super-elderly patients are unexplored. Finally, another limitation of the study is that the risk of death in the long term after discharge from the hospital was not assessed; further large-scale studies based on real patient data are needed. In conclusion, this study, the largest on hip fractures in the super-elderly, analyzed data from Japan to examine the sequelae in those aged 90 and older. Compared with those aged 65–89 years, the super-elderly had a significantly higher risk of pneumonia, urinary tract infection, acute renal dysfunction, subsequent cognitive dysfunction, and in-hospital mortality. Despite high 30-day survival rates (99.1% for the elderly and 97.9% for the super-elderly), in-hospital mortality was higher for those over 90 years of age. As this study has revealed that the risk of sequelae of hip fractures and postoperative mortality is high in the super-elderly, it is necessary to re-recognize the importance of fall and fracture prevention, osteoporosis treatment, and early surgery in the super-elderly population.