Jeff Williams, chief operating officer of Apple Inc., speaks during an Apple event at the Steve Jobs Theater at Apple Park on September 12, 2018 in Cupertino, California. Justin Sullivan | Getty Images News | Getty Images

As Apple pushes deeper into health care with the Apple Watch, the company is developing a plan to help people who are recovering from knee and hip replacement surgeries. On Monday, Apple announced a partnership with medical device company Zimmer Biomet, to combine a new app along with health-tracking data from the smartwatch to help determine why certain patients recover faster than others from the procedures. The companies are also working together on a clinical study. Apple has its sights set square on the $3 trillion U.S. health-care sector and is continuously exploring medical applications for the watch, most recently adding an FDA-cleared EKG sensor. When it comes to orthopedics, more than than 1 million Americans get knee and hip replacements every year, and Zimmer Biomet is among the biggest manufacturers of reconstructive products.

Apple and Zimmer Biomet have created a mobile app called mymobility, which aims to help guide patients through their surgery to improve their experience, as well as their health outcomes. It includes educational resources, exercise videos and a way for patients to contact their surgeon and care team with questions and concerns. The Apple Watch will track steps and heart rate data, allowing patients to share that information with their doctors to provide a clearer picture of how they're doing after surgery and to analyze potential setbacks. For example, if a patient is concerned about the level of pain, the care team could see that the patient walked five miles the previous day, so the problem may be overexertion rather than a serious complication. Apple and Zimmer Biomet are hoping to enroll 10,000 people in the U.S. in the study. The app is initially available only to patients who enroll in the study, but it will eventually be rolled out to everyone, a spokesperson for Zimmer Biomet told CNBC. Participants who don't own an Apple Watch will receive one for the duration of the project. Fitbit has also looked at using its fitness trackers to monitor patients after surgery.

Zimmer Biomet and Apple's Apple Watch app Zimmer Biomet

Zimmer Biomet CEO Bryan Hanson said in a statement that the partnership with Apple marks "one of the largest evidence-gathering clinical studies in orthopedic history." Apple Chief Operating Officer Jeff Williams said in a separate statement that the new app lets patients and doctors connect in a way that was "not previously possible through traditional in-person visits." The study is the second of its kind for Apple. In September 2017, the company kicked off its Heart Study in partnership with Stanford University, testing to see if it could detect in people a type of heart health irregularity called atrial fibrillation. Williams told CNBC in November that Apple is open to working with some of the largest health-care companies in the space and said, "We have taken a thoughtful approach to this and embraced the medical community."