One of the biggest X factors for the Miami Dolphins this season will be the health of starting quarterback Ryan Tannehill. The sixth-year veteran is coming off a sprained ACL and MCL in his left knee that forced him to miss the final four games of last season, including a playoff loss to the Pittsburgh Steelers.

What is intriguing is that the Dolphins decided surgery wasn’t needed this offseason and rehabbing the knee was the best way to go. However, no one, including Tannehill, is 100 percent sure how the knee will respond to the rigors and pounding of a 16-game season. Miami needs Tannehill, who had a career-best 93.5 passer rating in 2016, to be durable and play at the same level or better this season if it is to make the playoffs for the second consecutive year.

With training camp set to begin Thursday, we reached out to ESPN injury expert Stephania Bell for more clarity on Tannehill’s knee and what to expect from the Dolphins' franchise quarterback.

1. There was a lot of debate on the differences between an ACL sprain and partial tear when the injury first happened. Can you clarify to provide perspective on what happened to Ryan Tannehill?

Stephania Bell: There is often confusion as to what constitutes a sprain versus a tear, but the truth is they're technically not different. The term sprain references injury to a ligament, essentially involving tearing of the collagen fibers that make up the ligament. Sprains are graded 1 (mild), 2 (moderate) and 3 (severe). A Grade 1 sprain can be as simple as a "stretching" of the fibers without any visual tearing (microtrauma) or very minimal tearing. A Grade 3 sprain is a complete tear and a Grade 2 sprain is everything in between, meaning a wide range of tissue injury. Sometimes in casual language the word "partial tear" is used in an attempt to convey seriousness of an injury not always associated with the word sprain. But we are splitting hairs when trying to differentiate between "sprain" and "partial tear." Reports on Tannehill have ranged from "sprained MCL and ACL" to "Grade 2 MCL and partially torn ACL." They essentially indicate the same thing.

2. Are the chances for reinjury for Tannehill any higher from an ACL/MCL sprain without surgery?

Bell: Most MCL sprains are treated non-operatively, so the main question surrounds the integrity of the ACL. An ACL that is not normal has a higher chance of injury than a normal ACL. Ultimately when it comes to evaluating an ACL injury, the main question is whether or not the knee is functionally stable. The ACL is critical for overall knee stability, and the primary goal of reconstruction after an ACL tear is to restore stability via the new graft. Tannehill underwent rehab for his knee. The key goal is still that his knee is functionally stable going into the season.

3. Tannehill is one of the higher-profile athletes to get a stem cell transplant this offseason. What does that typically do for recovery?

Bell: Although stem cell therapy is becoming more well-known, the reported benefits when it comes to many orthopedic conditions still remain largely anecdotal. There is no definitive evidence (based on clinical trials, for instance) that recovery from ligament sprains is significantly altered or enhanced.

4. What are the advantages for wearing a brace?

Bell: Sometimes using a brace primarily helps with an athlete's mental confidence, particularly if he gets uncomfortable in a crowd where someone can fall on his knee. Quarterbacks who have been injured in or near the pocket often talk about having apprehension when they return to that pocket and have traffic around their previously injured knee. Some of the protection afforded by the brace helps with global knee stability. The side reinforcements of the brace help protect against medial/lateral displacement, which, for instance, helps protect the MCL on the inner side of the knee. The ACL is harder to control for because the rotational element that often accompanies this injury cannot be completely restricted with a brace.