When the running shoe begins to tilt inward (valgus) or outward (varus), it typically means that the midsole has started to become asymmetrically altered from the compression forces of running.
Not only is this a sign that it is time to get a new pair of shoes, but also that you should consider a different type of shoe. Continued running in those shoes could increase your risk of injury. The traditional thinking is that if the shoe is breaking down in varus, you need a shoe with less stability features, and if you are breaking down the shoe in valgus, you need a shoe with more stability features. Either way, it may be a red flag that you have some structural imbalances in your feet or legs.
Foot orthotics, particularly ones capable of completely balancing the foot imbalances (full contact custom-made orthotics), can often significantly reduce or resolve the issue, allowing you to wear neutral type running shoes and eliminating the development of shoe breakdown in valgus or varus.
New creases in the midsole are a sign that the midsole of the shoe is losing its cushioning, as well as a sign that early valgus or varus tilt could be occurring. Regarding the loss of cushioning, some people may tolerate this better than others; however, in order to determine if it is a significant loss, try on a new pair of the same model shoe and compare the feel. You may be surprised at how more cushioned the new shoes feel compared to the ones you are currently wearing, and this may suggest it is time to replace the old ones.
Outsole, or tread wear patterns can give insight into how you run. If there is significant wear at the heel, you are a heal striker. It is normal for some wear to be seen on the lateral side (outer) of overly-worn shoes under the 5th metatarsal (lateral forefoot) of a forefoot striker, and under the lateral heel of a heel striker. If there is more wear in the central forefoot, you likely need to replace the shoes, as although you cannot see what is happening to the underlying midsole, it is probably significantly compressed and you may be on the verge of injuring the ball of your foot from increased pressure. Also, if the medial side of the heel or ball of the foot is overly worn down on the outsole, you are likely running with an abnormal gait pattern and may be susceptible to injury. If the wear on the right shoe is not symmetrical with that on the left, you are probably running with some sort of compensation for a difference in your right side from your left side. If you are experiencing symptoms, this may give a clue to what the cause is.
Insole wear patterns, determined by looking at the indentions and wear locations on the insole of the shoe (most running shoes have a removable insole to allow you to observe this), can tell where the majority of your foot and toe pressure occurs.
The distribution should be relatively even under all places with exception of the arch area, where no wear should occur. The pattern should look like that of the “footprint” made when stepping out of the shower onto a dry surface. If there are focal high wear spots, this indicates a location of abnormally high pressure, and may correlate with a location of an impending or actual injury. Also, if the outline of your toe prints approaches or goes beyond the bounds of the insole, the shoe is probably too small, and resultant toenail or toe injury may result.
Bulges or worn down material in the upper of the shoe in the vamp region likely indicates abnormal pressures on the toes (top) or sides of the ball of the foot (medial or lateral sides at ball of the foot). In the toes, these could indicate the cause of an ingrown toenail, corns, hammertoes, ball of foot pain, and joint pain in the toes and ball of the foot. At the sides of the forefoot, this could indicate bunion or Tailors bunion formations, could lead to pain over these bones, as well as pain under the ball of the foot, especially burning pain that radiates into the central toes, known as a “Morton’s neuroma”.
Excessive wear about the interior heel region of the shoe, on either side, could mean you are developing a “bursitis” type condition (soft tissue pain from heel rubbing shoe) or conditions caused by over or under pronation.
You may have a bony prominence in the back (posterior) region of your heel which can cause such a wear pattern. Another cause may be your heel sliding out of the back of the shoe. If you are not experiencing symptoms related to this wear pattern, you may continue to wear these shoes with caution; however, if you do begin to experience symptoms, a change in shoe model should be considered.