The epidemiology of knee injuries in New Zealand, 2015-2024

Knee injuries are a common part of human life, occurring when the joint that connects the thigh and lower leg suffers damage from a fall, a twist, or a direct blow. While many people think of these injuries as serious breaks or torn ligaments that require surgery, the reality is far broader. Most knee problems are softer, less dramatic issues like bruises, sprains, or tears in the supporting tissues that often heal without ever seeing a surgeon. To understand the true scale of the problem, scientists need to look beyond hospital emergency rooms and operating theaters. They need a way to count every single injury, from the minor bruise treated by a physiotherapist to the severe fracture requiring a hospital stay. Without this complete picture, health systems cannot accurately plan for the care people need, and researchers might miss the most common causes of pain and disability.

In New Zealand, a unique system makes this kind of complete counting possible. The country operates a national, no-fault injury insurance scheme, meaning that if a person is injured, the state covers their medical costs and lost wages without needing to prove who was at fault. Because this system is the only provider of such insurance in the country, it captures almost every injury that occurs, regardless of how severe it is or where it is treated. Researchers used this vast database to track knee injuries across the entire nation over a ten-year period, from 2015 to 2024. Their goal was to move beyond the narrow focus on specific, well-known injuries and instead map the full landscape of knee trauma, seeing who gets hurt, how often, and with what kind of damage.

The study revealed that knee injuries are far more frequent than previously understood. Over the decade, the total number of recorded knee injuries grew from roughly 185,000 in 2015 to over 244,000 in 2024. When adjusted for the size of the population, the rate of injury rose significantly, particularly for women, whose injury rate increased by 22 percent over the period, compared to a 9 percent rise for men. The most common types of injuries were not the dramatic tears often seen in sports headlines, but rather soft tissue issues. The single most frequent injury was an unspecified ligament sprain, followed closely by bruises and collateral ligament sprains. These less severe injuries, often treated in community clinics or by physiotherapists, make up the bulk of the problem, yet they have historically been overlooked in medical research that focuses primarily on hospital admissions.

The data also painted a clear picture of who is most likely to suffer specific types of damage. Younger people, between the ages of 15 and 35, were most prone to ligament tears and joint dislocations. In contrast, cartilage injuries, which involve the cushioning pads inside the knee, became much more common as people aged, peaking between 40 and 65 years old. There were also distinct differences based on sex and ethnicity. Men experienced higher rates of ligament and cartilage injuries, while women had higher rates of bruises and dislocations. Additionally, while people of Māori descent had higher rates of severe ligament tears and dislocations, they had lower rates of sprains and bruises compared to non-Māori. The researchers suggest this might be due to barriers in accessing care for less severe injuries, meaning many minor cases among Māori populations may go unrecorded in the insurance data.

A striking finding was the sheer volume of injuries that were labeled as "other" or "unspecified." By 2024, these vague categories accounted for more than half of all recorded knee injuries. The researchers suspect this is partly because doctors in primary care settings, lacking advanced imaging equipment, often use broad codes to ensure patients get covered for treatment without the risk of a claim being rejected later if a specific diagnosis changes. This trend has grown over time, with the number of unspecified injuries rising sharply while specifically coded injuries have declined. This suggests that the true nature of many knee injuries remains hidden behind generic labels, making it difficult to know exactly what is happening to patients.

The study concludes that the burden of knee injuries on the health system and on individuals is much larger than previously thought. By capturing data from community care and not just hospitals, the researchers found a massive number of minor injuries that, while individually small, add up to a significant collective problem. The findings challenge the idea that knee injury research should focus almost exclusively on major ligament tears. Instead, the evidence points to a need for a broader understanding of all knee trauma, including the common sprains and bruises that affect millions. As the population ages and injury rates continue to shift, particularly among women, understanding the full scope of these injuries becomes essential for developing effective prevention strategies and ensuring that care reaches everyone who needs it.